Assuming the 4 speed is original to the truck, 1st gear is not synchronized and is so low that it is mostly used for pulling stumps. I hardly ever use 1st gear in my '52 GMC (same SM420 transmission), even though it only has 90 HP.
The rear tires are probably a lot taller than you are used to, so the 5.14 ratio is not as extreme as you might think. The center section of your rear axle will interchange with Chevy/GMC 3/4 and 1 ton trucks from the late 40s into the 70s, some of which have ratios in the high 3s.
I'd use the TH400 unless you really plan to use the truck as a truck a lot. I think your "4 ton gross" number is a bit high. 3 tons is more realistic.
The 389, with its higher torque at lower RPM would make for a much better truck engine than the 283. V8 GMC trucks from 55 through 59 were equipped with Pontiac V8s.
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My Pontiac is a '57 GMC with its original 347" Pontiac V8 and dual-range Hydra-Matic.
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