I am just starting a 63 longroof. Well, not just starting, I am also working on my 72, so little gets done to the 63. Just collecting parts at present.
A small block will fit (it's in my plans) with little or no mods. Bigger engines can be installed, but more modification is needed. The 348 would be an unusual swap, but probably anything could be put in there with enough work. An aftermarket front clip would make it easier. With stock front end, the rear steering pretty much dictates a front sump. Chevy20nly, among others, has complete swap kits (http://www.chevy2only.com/
Dan (another longroofer)