My truck sputters when going uphill, and has died a few times because of a lack of fuel getting to the carburetor. It has a new mechanical fuel pump and fuel filter, custom fiberglass tanks under the fender on each side, and I have replaced both gas caps. The carburetor has also been replaced with a Holly 4-barrel. It seems like it could possibly be a vapor lock issue, but I am not really sure what is going on.
I would only be speculating on this problem since you have altered the fuel tank system on the truck. I suspect that the fuel pump is getting starved of fuel and when you are going up a hill causes the pump to not be able to keep up with the amount of fuel needed to supply the carburetor with enough fuel. I would check the volume of fuel you are getting from the fuel pump and see if it meets the specifications of the pump. If it is too low then look at getting a high volume pump and even look at getting a low pressure electric pump to help keep fuel supplied to the mechanical pump. Put the electrical pump as close to the fuel tanks as possible. I would also recommend that you have a professional, certified mechanic take a look at it, such as one from YourMechanic.