I had the pleasure of working on a 53 for a customer. They had purchased conversion mounts to swap in a LA 318. I found them the manual trans setup from a Poly 318 out of a 65 D500. My job was to plumb and wire. Make it start and stop. You are correct on the actual size. They look huge, until you sit in it. People were smaller in those times! Lol! I too, thought a 12v Cummins swap would be cool. Firewall mods would be a must. Dang neat trucks! Good luck with it!!