518 and 500 are about the same in size externally, so you can use either one. The issue with the late b-bodies is the isolated torsion bar crossmember. I talked to one guy who did a 518 on a 73 charger, he got it to fit without modding the isolated torsion bar crossmember, but chopped the stock frame/ upper torsion bar crossmember. However since the 518 can be fit under a 71-72 B body torsion bar crossmember with some trimming of the ribs on the transmission, it's possible you may be able to get the 518 to squeeze between the upper torsion bar crossmember and the lower isolated torsion bar crossmember. On mine I am very much considering relocating the torsion bar hex sockets to the upper torsion bar crossmember and junking that isolated torsion bar crossmember. This will require going to teh 72 and prior t-bars but will get that pesky isolated t-bar crossmember out of the way and will allow me to install frame connectors.