85 S/Box Silverado Dual Fuel Tank Question

[Already Gone]

Last week me and a friend flew to LA to pick up a 1985 S/Box Silverado 305,4 bbl. A/T truck which we drove 1800 miles home. We made it but had a strange issue on the way home. Day 1 we drove from LA to Vegas without incident. Day 2 heading north up the I15 about 50 miles from Vegas going up a slight hill the truck started bucking and feeling like it was gas starved yet when we were coasting down a hill or on the flats or pulled over idling it ran perfect. The truck has the factory dual tank setup and we were only going to use the left tank as we knew it worked. As we drove the sputtering and starving got worse. When we hit Mesquite Nevada we were limping pretty good so pulled into an AutoZone store and asked ( this was Sunday of the Memorial Day weekend) if they knew of a garage that was open. There was no one open but a local young man has a mobile repair service and he came. He changed the fuel filter on the carb but that didn't help. As we were test driving after the fuel filter change and the truck was acting up the mechanic flipped the gas tank switch and the truck straightened out. The mechanic then suggested that is was probably the valve that is mounted on the frame that controls the flow from the gas tanks. He changed that for us and it didnt help as the truck still bucked and f**rted on the left tank but ran perfect on the right tank. We headed home using the right tank and made it with almost no issues. The truck started hesitating a bit going through the mountains in Idaho and Montana but never to the extent that we had to pull over. Over the next two days we drove exclusively on the right tank and when it reached the half full mark that is when the truck hesitated. I'm thinking the gas tank sending units are shot but my friend and a few friends we've talked to think its the fuel pump and my thinking on that is if its the fuel pump it would act up on either tank. Has anyone experienced this before with a factory dual tank truck?