I would go ahead and drain the tank, clean it out. If it makes a little bit of difference, it seems to be the constant that hasn't been fully addressed. It isn't that difficult to drop the tank, seems like you have all the other items under control. 5/16ths inch line replaced with 3/8ths would be a good idea, too, and being a truck, a lot easier to get under there and swap it out. To check if it is tank vent vacuum problem, run it until she dies, then listen for the suction on the cap when you open it as quickly as possible after she shuts off. If it sounds like a vacuum suction when you open it, try driving her again with the cap cracked a little bit and see what that does.