An educated guess would be that the D-20 would be a bit weak behind a 460.
I would think an Atlas T-case would be in order.
However., another issue would seem to be a nose heavy condition in a short wheelbase rig. Likely need suspension & other handling mods.
The mix-n-match experts to talk to would be Advanced Adaptors.
These rigs don't need a lot of HP unless you're runn'n deep mud, etc.