Matt167, Sounds like the same problem I had with my ranchero.
I thought It was vapor locking when it was really a venting problem. The car ran fine at 30 to 45 MPH but If I went faster then that the car would die. It was creating a vacuum in the tank. Ends up the vent tube was pluged up. I had tried to leave the cap off the tank But the fill tubes {ONE HOSE IN ANOTHER HOSE}where collasped on each other. I guess they would'nt let the air in to the tank. I had Replaced the fuel pump, all the filters which there was three and thats not counting the filter in the carb. All the fuel line from the metal tube on the frame to the fuel pump then to the carb. The filters did make me think that the last owner was trying to fix a problem too. Hope this helps you, dont know if you have the same problem or not.

~ Vegas ~