[Prowbar]

Thanks for the help, Ken.
Yes I'm overseas now.

Battery, cables and connections all check out. I actually made new battery cables and they are beefy. None of them run hot. Battery is good, an old Optima but still potent. Tests and checks out OK.
Battery ground cable actually hooks up to the engine and its contact is good.
I've rebuilt the original starter already and it is in good shape.

The 478 has a lot of piston to move up and down and it bogs the starter down. It was fine with the 305.
The 478 uses a bigger starter from the factory in combination with the SAE bellhousing. The Chevy high torque starter should help to crank it over.

Now the nice thing is that I won't modify the original GMC starter. I wouldn't want to if that was the case. The only thing that I use the GMC starter for is the original nosepiece. And it will not be modified. So I can always convert it back to original.

The Chevy starter is readily available and no problem if it's modified. The high torque starter has a larger diameter rotor so it should have more torque.

What I'm wondering about is whether the original GMC gear is shorter than the Chevy gear (both are the same 9 tooth gear) to compensate for the shorter nose piece and shaft length on the GMC?