Keep in mind that the original stovebolt radiator is a low pressure design. I don't know if you can simply throw on a high pressure cap and not spring leaks. So you probably almost have to change the radiator.
As far as the great aluminum versus copper/brass debate, it really does depend more on your personal taste than performance. There are advantages and disadvantages to each, and I don't believe any one is better than the other.
Copper/brass is more efficient than aluminum at conducting heat. However, the fins per inch have to be less, and you need more cores thickness to improve heat transfer. And it's true that the thicker you get, the more it follows the law of diminishing returns. You can go up to 5 core, maybe 6, without running into this rule of thumb, but start getting thicker than that, and each additional core is less effective.
The big advantage to aluminum is the really tight fins/inch that you can get, and the tubes are broader. So it's true that you make more efficient use of the core, meaning less core count to equal a thicker copper/brass radiator. This results in less weight, (which is not a big deal issue for a heavy truck anyway). I'd estimate that a 2-core aluminum radiator is as efficient as maybe a 3 core copper/brass radiator. However, when you get up into the 4 or 5 core copper radiators, I don't think the aluminum cools as good as those.
...And this is by experience. I easily keep two big block engines cool with 5-core copper radiators, yet the fancy schmancy 2-core aftermarket aluminum radiator in my 350 Camaro barely does the job!!!
