It will not fit into the '54 6100 without a severe cut in the firewall and that will leave your toploader tranny stick shift somewhere behind the cab so you'll need really long arms and no back window if you plan to use it. Alternatively, you could remove the radiator and shove it all the way forward and then mount the radiator somewhere else with an electric fan, but I still think you'll have to carve up the firewall to make it fit even without the radiator.
Cranks are not made and are about the hardest part to find for these after the camshaft and block. These were a steroidal evolution of the big block 351 GMC V6 engine.
Here's a video I took two years ago of a display V12 at the ATHS National Meet in Hutchinson, Kansas:
And here's the one that our oldGMCtrucks.com forums member "Locomotive Breath" built for his truck.
Of course he's also got a Mustang drag car with a tank engine in it, so putting big long motors in small spaces doesn't seem to concern him at all!
