Drivetrain help .. Where to look first?

[C10Silverado79]

My truck runs pretty well but has no real power. I don't think I could break the rear tires loose on a wet painted crosswalk. The other problem I have is when I try to tow my boat. As soon as I hit a decent upgrade, the motor bucks and coughs like it has no fuel. Here is what I know about it:

79 Silverado
350 CI
350 Auto trans
Q-Jet 4bbl (I rebuilt it last year but no guarantee I did everything right)
Rear ???? (no idea)
Stock Exhaust manifolds
Y pipe - single exhaust
replaced:
Plugs, wires, cap, rotor
Fuel Filter
rubber fuel lines back to tank.
Fuel Pump
Vacuum Lines
Advanced Timing some. (might be almost too advanced)

Maybe it's just the design but I feel like when I hit the gas, the motor revs but the power seems to just "go away" and not reach the rear wheels.

Any ideas, thoughts, suggestions? Could a partially clogged CAT cause this type of behavior?