Why would it bog badly in a truck? How would the the engine know what vehicle its in? I helped a friend put a cleveland in a 79 shortbox stepside 2wd. That engine was ********. Never bogged at all, that thing would fry the tires all day long. If a 302 will move a truck a 351C will be just fine.
Which 351 Cleveland did you install in a 79 stepside? There are 3 variations of the motor. A mild performance 2bbl 351C with the same size ports and valves that the 351M/400 has, a 4bbl 351C with 2.19"x1.78" valves and huge ports, and 4bbl BOSS 351, which has the same heads as the standard 4bbl 351C, but with smaller quench chambers for more compression, and a hotter solid lifter cam.
The 4 bbl Cleveland has intake ports almost the size of tennis balls, and intake valves 2.19 inches wide. The motor bogs in ANY vehicle, unless you rev it, but the heavier the vehicle, obviously, the worse the bog becomes. The reason it bogs is due to the fact that the ports and valves are so enormous, that they **** velocity. When you lose velocity, the carburetor doesnt get a proper vacuum signal, which inhibits the main jets from pulling fuel. (That means low end torque doesnt exist.)
Once the rpm picks up, intake velocity increases, and the motor begins to make power. Sure, it would go like **** in a properly set up truck. (By proper, I mean deep gearing and either manual trans or auto with high stall speed converter) But for any type of pulling or heavy use requiring low speed torque, a 351C 4bbl would pretty much ****. It's not just NOT a truck engine, it's the polar opposite of a truck engine.
The 351C 2bbl, on the other hand, as I mentioned previously, would make as good a truck motor as a 351M, since it has basically the same displacement, bore and stroke, heads and intake. The ports and valves in a 351C 2bbl motor are nice, they're big enough to make power without being so huge that they **** low speed torque.
P.S. I wouldnt put a 302 in a truck either. A 3" stroke has no business being anywhere near a heavy vehicle! Although some of them run ok if set up and tuned just right, it's still not what I'd consider a very desirable motor for a truck, unless it's a seriously built 302, and it's a lightened drag truck.