Help wanted for power loss under load...

[C10Silverado79]

I have a 350 with a Q-jet 4bbl. I rebuild the Q-jet a couple of weeks ago and the truck has been running very smoothly. I haven't noticed any issues at all till Friday when I went to pickup a new Pontoon boat. Aside from having the aerodynamics of a brick, the boat and trailer only weight about 2000 lbs. The truck ran fine when with the boat attached until I started hitting some lengthy hills. Even though the hills were not real steep, the best I could do with the trailer was 50-55 mph. The big concern was when I hit WOT or close to it. The motor would stutter and surge like it was starved for fuel. If I backed off the throttle a bit it would calm down a little but would start acting up again if the speed dropped too low and I tried to accelerate. It even did this to me once on a slight down grade. Just lost power all of a sudden then smoothed out and was ok. The motor never stalled but a couple of time it literally would act like it stalled, the would surge, the 'pause' then surge then 'pause'. Over the past couple of months I put new plugs, wires, rotor, cap, fuel pump, water pump, hoses, belts, brakes, rebuild carb, vacumn hoses.

I'm not really sure where to look for the problem. I have no issues when not towing a load except that the I don't think the motor is as strong as it should be. Any ideas will be appreciated.

Thanks
Steve