There's a lot of info on the forum here on how to do it.

Basically the hardest part is mounting the trans. Space wise there's not much difference between a 500 and 518. Both can be built to take a decent amount of HP, so it boils down to which one you want. IIRC the JW ultrabell will bolt a big block to a 518 but not a 500. E body and 71+ B body don't require torsion bar crossmember or floor pan mods. 1_wild_rt posted pics and instructions on this several years ago. Older b-bodies to require the crossmember to be trimmed. The mount to hold up the trans will have to be custom fabbed by you. Torq converter selection is a little tougher with the lockup 500/518, however the lockup is nice for cruising and keeping the rpm down. A non-lockup 500/518 will use a regular 904/727 converter.