MAER,
I looked at an exploded view of your carburetor but it isn't clear to me if there is any internal filter beneath the carburetor inlet. I think this is the one Jerry (HRL) mentioned changing.
If you are dead certain that the octane of the fuel made a difference, it would tend to make me suspicious of your HEI unit, or at least some facet of the ignition system.
Have you tried running the truck up a steep hill and lugging the engine a bit. I don't know what kind of gearing you have, but can you get the rig above this problem, or does it get worse and continue to buck and cut out?
It isn't clear yet (to me anyway) if the problem is the ignition or a starved carburetor.
Stuart
