You have a heavy truck. You only have a three speed manual transmission. That kind of limits how much you can do to improve the horsepower. Here is the dirty little secret. To increase the horsepower, you shift the speed that the engine makes torque to a higher RPM, and that higher RPM gives you more horsepower, but it robs torque from the lower RPM range.
Reality is you would be better with the 300 inline 6 cylinder, than the 302.
However, there are improvements that can be made. But do not go too wild on the cam, or the carburettor.
Maybe do a leak down test on the engine to confirm the rings are really in good shape. There might be some gains from pulling the heads, and increasing the compression, but it would be easy to do too much, and end up having to run a mid grade of gas, or even a premium grade, and at the mileage a truck gets, spending an extra 25 to 40 cents a gallon might not be a good idea.
A valve job might not be a bad idea. You might want to see if the heads have hardened exhaust valve seats, in the 1970's gasoline still had lead in it, and the exhaust seats did not need to be hardened, but without the lead, now they do.
Start out with a budget. DMWYT. (Don't Mess With Your Transportation)
Another idea would be is to get a second 302 engine, and do work on that engine, so you have a running truck. It should be pretty easy to find a 302.