The newest engines of that series are close to 50 years old, and they were not produced in very large quantities. I can think of several better choices for repowering your truck, and tractors need a totally different kind of engine- - - -one with a governor, lots of torque, and the ability to lug down when the going gets tough. The GMC V6 has enough torque, but I'm not sure how difficult it would be to find a governor that would work with it. It's also extremely heavy for the amount of power it produces. Good for restoring an original V6 truck, not so much so as a swap choice.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!