[izzy_Britton]

i can only speculate as to what i have seen on many of the other builds and what i can remember. but it seems to me that the 4l80e isn't much different in diameter (height) than the the TH400, the TH400 fit fine under the cab so the 4L80e should fit fine as well. i don't recall off hand any builds where the floor had to be modified for the 4L80e to fit unless the engine was moved rearward. so removing your cab shouldn't effect that portion of the install.

also the LS in the stock SBC location will actually give you slightly more firewall clearance because there is no distributor to interfere with the firewall, typically that was an issue in the past with distributor equipped engines.

i cannot answer anything about the header question, i am not a header fan. I like the fit of my factory camaro exhaust manifolds. did/does your purchased engine come with manifolds? if so you could use them. there may be a slight frame clearance issue but most people just notch the frame a hair to get them to fit in there.