The thing to remember about Wilberforce is that he was a)British, and b)living about 200 years ago. Even today, what British Christians call Evangelical and what Americans call Evangelical, while having theological similarities, have many practical differences (which are probably not relevant to this discussion). Evangelical Anglicans are a very significant force in British Anglicanism, but they're not necessarily what an American would think the word means. There's also of course, the fact that 200 years ago, Evangelical in America didn't even mean exactly what it means in America today; as you say, this is before the modern evolution of "Evangelical" vs "Mainline". (I'm not going to go into the exact details because I think it's not particularly relevant to this question.)

Now, the difficulty with talking about American denominations is that many of them have changed, reformed, etc, since this time, and many Evangelical Churches today are actually schisms from mainline Churches that happened in the 1960s or even 1930s. Multiple Presbyterian denominations split from more mainstream Presbyterian churches, then split themselves. Most of what would be considered Evangelical Churches in the 1860s, are now what would be considered Mainline Churches, because the more conservative elements left to form their own Churches. America now has a bewildering array of denominations, and tracing them back to pro- or anti-slavery Churches is a bit misleading; most of them are descended from both pro- and anti-slavery Churches.

However, there are a few general trends that can be noted. Quakers had been anti-slavery for a long time. Methodists began to become anti-slavery, but fearing a split the Church leaders didn't want to take a stand; this led to a split, as some Methodists left to form an explicit anti-slavery Church, while some Southern Methodists split to support slavery. Baptists too, were more likely to be anti-slavery, but those who lived in the South, if white, were much more likely to be pro-slavery. This eventually led to the split with the Southern Baptists, who did leave explicitly because of slavery, and are one of the larger denominations that Americans today call Evangelical.

A good example of the splitting and reforming is the Presbyterians. The Presbyterians began to split in the 18th century over theological divisions (the New School/Old School). But when slavery became an issue in the 19th century, both the conservative Old Schoolers and the less conservative New Schoolers split over slavery. So here we have an example whereby there are more liberal theologians supporting slavery in the South, not just those with a "sola scriptura" reading. Actually, the Old Schoolers took longer to split, as they had the view that it wasn't the Church's job to support or attack social phenomonen (like slavery). Much later, these Churches reunited, but there were splits in the 1920s and 30s over the Fundamentalist/Modernist contraversy (things like "Was Jesus really born from a virgin", "is the Bible fallible", etc). It's that contraversy that really fuelled the Mainline/Evangelical split, not slavery. Now, over time, what Americans call Evangelicalism became more popular in the South and less in the North, but this is really a development from the 20s to the 60s.

Tl;dr: Generally speaking, both Mainlines and Evangelicals are descended from both anti- and pro-slavery Churches, as the schism was largely about north and south, and was mostly later healed. But there are a few particular Southern Evangelical Churches that are descended from pro-Slavery churches, like the very large, very conservative Southern Baptists.

Great question. I look forward to an answer and I would like to add: if we can't pinpoint which denominations supported abolition mid 1850s then what were the prominent denominations in the south compared to the north? Was it Baptist vs Methodist? Could it have even been forms of Wesleyan vs forms of Calvinism?