I've seen a few /6 powered Cudas and Challnegers before. Rare sites indeed.
All S/B V-8 have the motor mounts in the same spot. Theres a slight naorrowing difference with the 318/273. If you swap in a 340/360 inplace of a 318, a few washers can make up the space, so new mounts are not needed.
you can use any S/B V8 of any year until the Magnum engine came out. The first few years of Magnums have the "LA" mounts on the side, later one do not. Crate engines have dual mounting locations on them.
Swapping V8 inplace of a /6 is a mystery to me. I beileve Schmaucher has an answer though from what I know, the K frames are all the same, you just need the right engine mount to go into the K frame.
I just don't know if that is the case with the /6 in a E body.