Do you guys have any specific facts that I could bolster my argument with?

As a general rule, you should gather data (in your words "specific facts") before you decide what your thesis is. Not that your thesis is a big problem (though I've never seen manufacturing described as "relentless" before), but you're thinking this task through backwards.

Also, given the vast universe of information that exists about the Second World War, you should probably stick to the materials addressed in your class.

A good book to use for your research is Richard Overy's "Why the Allies Won". It's a broad overview including production, natural resources, tactics and strategies, politics, etc.

Given all the territory it covers, I can't summarize everything, so I'll choose one of my favorites: war production. It explains why Germany, despite substantial industry, was unable to produce enough useful military equipment. Much of it is political. Before the war and in its early stages, the military played a large role in running the industries, micromanaging as we would say today. That was a task they were not well suited for. To make it worse, the German military loved the idea of carefully "craft built" weapons, but seems to have overlooked the importance of sheer quantity available though mass production. The situation only improved after Hitler appointed Speer as armaments minister, but even then it was an uphill battle. Nevertheless German war production increased throughout the war, which some people take as evidence that allied strategic bombing was ineffective. Disproving that is hard, because it's a counterfactual, but he does a good job of demonstrating how much the bombing hampered German production. Speer's post-war testimony to the effectiveness of the bombing also buttresses that argument.

Another problematic aspect of German production was their tendency to concentrate on super-weapons and ignore mundane necessities like trucks. Even at its height the German army was heavily dependent on horse drawn transport. By contrast the USSR, for example, made good use of the 400,000 trucks that the US shipped to them

He also describes US and USSR war production, and contrary to stereotypes, points out that they had many things in common. For example, both eschewed micromanagement and told organizations what to do rather than how. This led to surprising innovation at lower levels, and contrasts starkly with the situation in Germany.

Definitely look at lend-lease aid for both the manufacturing and cooperation aspect. Most lend-lease aid to Russia was support equipment like trucks trains and tractors. This was because the easiest route to Russia was across the Pacific in Russian ships. Russia and Japan did not go to war with each other until until spring of 1945 so Russian ships carrying non military cargo would not be intercepted by the Japanese. The Japanese unwillingness to declare war on Russia is another example of lack of cooperation.

Russia had it's own geographical barrier in it's sheer size. But also remember geographical barriers are double edged swords. At one point the Germans were sinking allied ships faster than the allies could produce them. If that attrition had kept up the untouched U.S. factories would have been Useless. The same barrier that kept the Germans from invading Britain kept the Allies from opening a second front until 1944. The large expanse of Russia proved perfect for German tanks to penetrate deep behind Russian armies and encircle them.

Consider changing relentless manufacturing to mass manufacturing. The Germans produced lots of equipment that was superior to the equipment used by the allies but this equipment was often complex to build and maintain. The allies on the other hand tended to produce mass quantities of equipment that while inferior to the German equipment were effective and were easier to produce and maintain.

Don't take my word for any of this though. I recommend you look it up and draw your own conclusions.

Like others said you should have the facts before you form your argument.

The Allies won the war because of their relentless manufacturing

While this is somewhat correct it should be better to argue the allies had an advantage in ressources and workforce which allowed higher production. I think it would be a big mistake to talk about manufacturing as war winning factor if you don't talk about man power aswell. I would argue your thesis is severly flawed if you disregard this.

the superior cooperation between allies

True but its debatable if this had the same significance than your first point. Regarding your wording: while the allied cooperation was far superior to the axis you have to show that a better cooperation between the axis would have had a major impact. Otherwise you should frame it differently.

and the geographical barriers that protected not only the US, but Britain.

True. Interessting point. You should add the huge space, the soviets were able to abandon without losing the capabilities to supply their army, when you talk about geographical barriers.

Do you guys have any specific facts that I could bolster my argument with?

The Gdp's should help you with your first arguments. The natural ressource production aswell. These numbers are available online.

Harrison, Mark, "The Economics of World War II: Six Great Powers in International Comparison"

In the long run it showed that oil was more significant than other natural ressources. Arguing the advantage in ressources won the war could be explained with the example of oil. Germanies airforce for example had more problems with shortage of fuel and pilots than with aircraft. The same is true for the armored forces especially the tank forces. This would disprove your argument to some degree.

The relentless manufacturing angle might be a bit off, considering that Germany's industrial output actually increased despite of the strategic bombing campaign. What was relentless was that the United States had the oil industry that could actually power their military vehicles, and transport the industrial output to the front lines.

And that's where the cooperation part comes into play, as the Allies were able to share technology to keep those logistical lifelines open. The development of radar, better sonar, using mathematicians to formulate the best ASW formations. In contrast, the Germans were relying on centralized synthgas facilities and a limited amount of crude oil from Romania - once those targets were neutralized there was a ripple effect throughout the entire German war machine - pilot training was curtailed, and ground forces could only burn fuel in battle (which says nothing about how they actually got themselves to that battle - farm animals).

But, you might only want to use that angle if your class actually touched on the logistical war. However, if you do go down that route, I'd suggest reading Jimmy Doolittle's autobiography as a good tie in to the more popular notions of WWII history. His prewar prodding of Shell to come up with a better aviation gasoline led to the first shipment being ready for the RAF shortly before the Battle of Britain. He also later presided over the 8th AF campaign to knock out the German synthetic oil plants. Really goes to show how much the Americans realized that the war was about energy.