A chevy 261 would be top on my list. Use the 42 bellhousing and mate a t5 transmission to that combo using an adapter. Plenty of power and fuel economy can be had. The 42 3/4 ton has a modified 1/2 ton rear. the lower gears can be swappped to 3.38 or 3.90.

In my 1940 3/4 ton gmc, I used a 302 gmc 6 cylinder with a dodge NV4500 5 speed, and a full floating 9 inch rear. I will have disc brakes front and rear, and retain the stock wheels and lug pattern.