A518 in a 68 Dart?

Mopar Action did an article on this swap a few years back. The crossmember needs to be modded a bit as does the floor pan. The rear crossmember carries the torsion bar load so you need to put it back together. I think all A500's were lockups, so converter selection wont be as good.

Another option which doesn't require crossmember work is the Gear Vendors unit. Unfortunately they are kinda spendy.