Is this the truck?
Pictures

http://oldgmctrucks.infopop.cc/eve/forums/a/tpc/f/4101021591/m/47420793073/p/1

If so then vcrash, are you the newest owner? Sounds like the last owner knew it was a 4-51, did you get that confused with a 4-53 or did he not tell you what it was to start with?

In any case, it would be a swap.
I have seen a service manual for a 4-51 in a Chevy truck, the manual was in Spanish and apparently if any of the trucks sold they were probably not in the US, I've never seen or heard of a 4-51 in a truck (from the factory) other than that manual.
Looking at the pictures it uses the factory rear mounts on the GMC crossmember, if it has a flywheel housing that does this cleanly and not cobbled together then the engine could be one of those intended for a GM truck. Or even sold by the Detroit dealer new to swap into a GM truck. This would be quite rare.

All said and done it's a 4-51, pretty uncommon and parts are extremely hard to find in good condition. It's basically a novelty engine at this point, not something I'd want to depend on.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-