Not sure if you have access, but if you have another carb, might want to pop that on to see if there is a large difference. Maybe a friend's carb? That'll tell you if it's a carb problem, quickly.
I had lots of problems under load- I went for the timing/carb first- it turned out to be either the plugs, wires, cap, or rotor. Changed all of them at once on a complete whim, and bam, fixed. Went from a almost unusable car to perfect.
I hate these problems that we work so hard to fix, and they don't go away- I've started taking the 'bazooka' approach- replace whole systems (carb, ignition) to quickly see if the problem is contained within.