[ted]

Putting a 410 or 428 crank with it's 3.98" stroke in your 390 would be a big step up in the torque department. This would provide you with a 416 cubic inch combination if only bored 0.030" over. Also adding an older Edelbrock SP-2P intake manifold with its very small runners will also be step increase in the low end torque department. This manifold starts laying over at 4500 rpm so it would be perfect for your combination. I still see these manifolds floating around at various swap meets and are usually priced reasonable to cheap due to its limited rpm capability.

I've already ran this combination in a '65 3/4 ton pickup that was used for pulling a trailer and can attest to its low end grunt. The fuel mileage was also surprisingly much better than the 352 it replaced. The 3.98" stroke crank also likes less gearing with the above combination. Your 3.50 gears would work nicely. My effective rear ratio with gear splitter activated was 3.19:1 with the 4.10's in the back being very effective for takeoffs with heavy loads.

Your small valve, small port heads are already suited for your intended use and the ports match the SP-2P intake in their stock form.

If money was no object, then I'd really consider the SCAT 4.25" stroke crank in the 390 which would net you 445 cubes with the same 0.030" overbore. Then the stock intake manifolds could be used for the same effective runner volume as the SP-2P on the smaller engine with the still monstrous increase in low end torque. I still like aluminum intakes over any cast iron version though for all the obvious back breaking reasons.