I LOVE IT!

You sure are going to get tons of engine opinions. I would love to see you stay with the poly 318, but can't answer your questions on availability of parts. I had a poly 318 a long time ago in my '67 utiline, three-on-the-three and a 4.10 rear and it was stout!

I would consider keeping it gas powered small block to keep the weight down up front, mods to a minimum, and costs moderate. I think if parts are available rebuilding the poly 318 would be great! Looks great too just like you wrote, and just belongs. Otherwise, I'd consider a stroked 360. It would have plenty of power to move that beast, carburated or FI, fit nicely, sound great, and have tons of parts available. IF IT WERE ME, I'd keep it looking stock Mopar, or at least as much as possible, e.g. valve covers, air cleaner, engine paint, etc.

Again, I love the truck, great score, keep us posted please!