Hey Jack. Welcome to classicbroncos!
I can't say with 100% certainty of course, but at least part of your trouble is likely the location of the pump. As it sounds like you're leaning towards.
Electric pumps (especially basic ones) don't like to pull. They much prefer pushing and are much better at it. Over the last 40 years or so I have only heard of a handful (or less) of people who've had even remotely decent luck with a pump mounted in the front of the truck. The only way to really do it right is to mount it towards the tanks.
With dual tanks, the "easiest" if that's possible, would be to put it right in front of the valve under the seat. But mo betta still would be to mount the pump somewhere in-between the two tanks, then run the valve outlet hose back to the pump, then back to the engine. Short of using two pumps, one per tank, this would be the best.
The only other thing that comes to mind is what some do with two pumps. That's to use only the main tank with the electric feed pump, then another small cheap pump used to transfer fuel from the side tank to the rear tank.
Some do that when their selector valves fail, but most that are doing the transfer method are running EFI with a single high-pressure (and more expensive) pump on the frame rail back by the main tank.
But no matter what, get the pump back as far as you feel you can. It can only help the pump stay primed. If all you do is get it down to the level of the tanks, you're on the right path. But if you can get it down low AND back as far as you can, better still.
If you don't drive the truck much it would be entirely possible that the first pump was just getting weaker from the new modern gas and sitting. Only new EFI pumps seem to be able to handle that for very long. But since you changed the pump to a new one and it's acting the same, then I'd have to say it's all about location, location, location...
Good luck.
And again, welcome. Post up some pics of your rig sometime if you can too.
Paul