Actually, 241 Dodge hemi motors were installed in those trucks from the factory at some point in the production cycle. The Hemi motors were available in the big trucks. The available info isn't real clear, maybe the early Hemi was another option above the early poly, or maybe the hemi was the original V8 offered but later Dodge changed to the poly as the standard V8.

At any rate, the early V8 motor & trans mounts were very similar to the flathead 6 motor & trans mounts and may have been the same mounts. I never pursued using the factory mounting system because I was installing a much more modern drive train.

My son bought a 56 Dodge panel that had a 383 Mopar big block & 727 installed with an aftermarket motor & trans mount kit (Cal Custom, or maybe Competition Engineering). That panel was assembled in the early 70s. They hacked out the firewall from about the gas pedal all the way to the right side of the firewall and put in a piece of sheet metal. It was done pretty crudely. The 383 barely turned over and the guy wanted to keep it, so we installed a 360 with a 727 into it, and welded in a firewall that more resembled what would have been closer to original. My son drove that panel all through high school.
I believe the main reason the guy hacked out the firewall was because the 383 still had a car oil pan on it and it sat pretty far back. Gene