This sounds like an interesting project. I've been running a 270 GMC in my pickup since the mud '70s. Same as a 302 with a smaller bore. Yours being military raises a question about the distributer. The sealed military unit is quite different from the civilian one. The civilian distributer is the same as 235 &261 Chevy sixes. The military plugs and wires were different too. It is likely that this was changed when the engine was installed.
Our O'Reilly store here has plugs, wires, points, condenser, cap & rotor in stock. Wires should be long enough for the GMC. Ypo would have to order a rebuilt distributer but they have it. Ir the rear main won't leak more than a quart or two in the 600 mile trip I wouldn't change it now.
Though I'm GMC guy in your case I'd find a later injected 300 Ford L6 and transmission ('94-'96). Check these out. They produce lots of low end torque and with the proper gearing you would anyways be in the torque range the engine likes. You don't really need an engine that produces it's torque at an rpm that you won't reaching. It might fit the engine bay better than a V8. Then there's always a Cummins?
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