When I read papers by Thomas Jefferson or Benjamin Franklin or George Washington, they use very antiquated words and seem very grandiose or wordy--but they're still much more understandable than other, more archaic English (thou, thine, art, etc.). Is the way these men wrote similar to their spoken conversation at the time?
Furthermore, while I understand (but wouldn't use) most words in their vocabulary, do we commonly use any words that just didn't exist (in their modern sense) back then? (Other than the obvious things like technology).
tl;dr- I can understand their words in writings, but would I understand their speaking, and would they understand mine?
While you wait for an answer to those three questions, you might try reading the answers to this related question from nine months ago:
Probably some time in the early 1500s, though the late 1400s might be possible with some patience and concentration. There might be some head-scratching over pronunciation, multiple meanings, and figures of speech but you'd be able to recognize what you heard as English.
This may be a linguistics question more so than a historical one.