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?

[INSIDIOUS '86]

about 10*-12* initial timing is fine on a sbc
your rear gearing has alot to do with towing with ease
stock exhaust is just plain bad for anything but a daily driver
and emissions equip and cat kill some power as well
how old id the engine? tired engines blow more smoke than get moving figuratively speaking

[C10Silverado79]

The motor is original with about 84k on the odometer.

[INSIDIOUS '86]

ok well do you have a budget on parts if you need em and do you care about gas mileage? an overdrive trans that has been mildly built and some 3:73 gearing will get you the biggest improvement without killing hwy mileage outright and keeps it ok in city.

[C10Silverado79]

I don't have a budget yet. Trying to determine what the problem areas are and what it it will take to get it where I am satisfied. New motors, trans, rear are easy answers if I want to invest the $$$. I'm just not sure yet what direction I want to go or exactly what where the performance drawback lies.

[J Knight]

Clogged exhaust will do exactly what you descibed. I doubt your motor is tired with only 84K (if thats original). I would dump the Y pipe and cat for duals, set timing and adjust carb. To me that the most likely suspect.