You have, then, with the 3.55 posi, already done the best thing you could do for your 248 GMC. Before pursuing a dual manifold, you might try one of Tom Langdon's progressive two-throat Holley-Webers on an adapter with your stock intake manifold. You could split the exhaust manifold or, if you want to spend the money, buy or make a set of headers; either would help the engine breathe. Tom Langdon also sells a "mini-HEI" ignition for the GMC--no more points, and a hot spark all through the range.
A dual intake manifold for the GMC, such as an Ellis or McGurk, did not come from the factory with heat. In Nebraska summers, that won't matter much, but in winter you will need heat sorely. If you can weld aluminum or know someone who can, then it is possible to add water heat to one of these manifolds.
For your stated purposes, with a recently rebuilt 248, i should not want to touch the head or install another camshaft. i think you would see little improvement from a stock 302 cam for the time and money invested. When the time comes that you want serious power in your truck, find a 302 and invest your money and labor in it. Your effort will be rewarded.
God's Peace to you.
d
Inliner #1450
