There are a few possibilities that come to mind-
Often high RPM break down and inability to pull at higher speeds is an ignition problem. Check the cap for cracks, the rotor and contacts for being in good condition, the coil for possible failure, the condition of the wires etc. If your car is popping and backfiring under load, I would start here
your fuel could be boiling in the float bowls. Often a heat shield under the carb will eliminate this. A simple cheap way to test for this is to wad up aluminum foil and tuck it under the float bowls. If this works, you know a heat shield is going to help.
The other is fuel starvation. You may want to check the pressure developed by your fuel pump and it would not be a bad idea to remove the sending unit and check it's condition. The fuel filter/sock on the end of it could be clogged. If the pick up line is cracked inside the tank it will suck air. when you are below a 1/4 tank you can jack the driver side of the car up as high as possible, put it on jack stands and remove the sender without draining the rest of the fuel.
Also your car should have a vent line from the top of the tank, up to the firewall. Often the charcoal canister is removed. Some people plug the line which doesn't allow the tank to vent. If this line coming off the firewall and pointing to the passenger side is plugged, unplug it, open the gas tank and try and blow air through it.