What year? The early 65 and earlier bodies are a real problem. The '66-later should be no problem if you order the proper headers for the engine/body combo. The two later B bodies I owned were 69 and 70, and it's been awhile. Seems to me that you did have to jack up the engine a little bit on one side, probably remove the starter when slipping the headers in there.

What I'm saying is, you may need to juggle a few things, but you won't have to hack up the body and you won't have to pull the engine out.

Different brands of headers will fit different, and whill have their own individual problems with plug wire routing, etc. On my '70, which had Cyclone headers back then, I welded up a special wrench for one rear plug. Even that was only needed for the strip, when the engine was hot. I could change plugs quite easily when the engine was cold.