Josh,
Just rebuilding a motor to stock specs is going to net you what the stock specs in your shop manual say. There would be no significant increase in performance unless the engine was on its "last legs". Stock specs are 123HP for the '55 235, and 148HP for the '56 261 for comparrison.
Both engines can be bored and stroked per Tom Langdons article, you can also increase compession with custom pistons, an 848 head, milling the head, or filling the chambers. Porting the head and using larger valves and camshaft can increase flow, but unless you're willing to improve the induction and exhaust systems, most of the potential of these internal mods will not be realized.
My opinion would be to leave the stock truck stock, and use the money to "build up" the other truck. If you want it to do highway speeds, adding HP isn't what you really need, its gearing. Since it is a '55 with an open driveline, I would suggest getting a set of aftermarket 3:36 gears from Patrick's, Randys Ring and Pinion, etc...
The GMC engines are definitely better for HP, but unless its a 270H or 302 (because of port size), I don't think I could justify the extra cost over the 261, in your situation. Just my opinion.
