From what I understand the blocks are all the same from an external standpoint aside from some bosses being drilled on the side of the block and some not, depending on what it came out of. I don't think it really matters though since the only holes you need are for the engine mounts, and I think those are all drilled on every block. The 5.7 and 6.1 are the same on the outside too from what I've heard, the only difference is the bore on the inside and some little things like piston cooling jets. I think the main fitment issues people have are the front cover and intake, but I'd wait to see what someone else has to say.