I would have to believe what you're after is torque. You have a heavy vehicle with a small engine by today's standards. I would aslo guess that you have a very high-geared rearend, since you can cruise at 75-80 easily. Do you have a tach? If so, what is your rpm at 65 mph?
The type of modifications you're looking at will not likely get you the performace you're after. A better flowing exhaust will make a little more power by itself, but the seatofthepants'ometer will probably not notice a difference. Swapping to multiple carbs or a larger single will expand your rpm range, but not gain you anything where you normally drive. It's far more likely to create a hesitation or bog within your normal driving range.
You may be looking at a gear swap of some sort to solve this. You may want to investigate a 700R4 trans with it's low first gear plus overdrive.
Hope I've been of some help
Devin
