You posted while I was typing. Do the tanks how you want, that was just my 2 cent up there. I did miss-understand what you were trying to do. By the time I got to the end of the thread I thought that you wanted to add an other aux tank and use the existing one as a transfer tank.
So am I right thinking that you want to keep the main tank, change the way that the aux tank feeds, and possibly add a third transfer/aux tank?
Carbed vehicles typically used a simple switching valve. Either mechanically or electrically operated. Trucks like my old F-100 would have had a simple ball valve mounted down on the floor by the seat to switch tanks, but that was also a mechanical engine drive pump.
My Ranger is an 85 dual tank, so similar vintage, but EFI. The Ranger has an electrical switch on the dash that controlls the tank seelction boy powering the selected in tank, low pressure pump. There is a filter/reservior that accepts feeds and returns to each tank, it probably has check valves to prevent cross feed, and a high pressure pump that pushes fuel from there to the engine. The reservior is necessary to keep the high pressure pump supplied when switching tanks. The dash switch also switches the gauge between the main and aux sender.
Your carbed truck would not have needed the reservior and it appears that they used a switching valve very similar to what
@bobbywalter posted above. Based on pictures of the valve and sender for your truck, it appears that even the carbed engine would have had a fuel return port which is surprising to me. If I had to guess, a dual tank version off your truck wouldsend power to the selected tank's pump and change the sswitching valve via a single switch on the dash.
If you can add a low pressure pump to the aux tanks (in tank or inline), a valve like the one that Bobby posted would probably be your best option for in cab tank switching. It's got two sides (one for feed, one for return) but if you don't have a return then you don't have to use both. Simple toggle switch on the dash will handle tank selection and a relay for switching between the pumps. For the second aux tank, add another valve plumbed into the first and nother relay to switch the power between the two aux tank pumps. . If I were doing it I'd have one switch/valve to go between main and aux tanks, a second switch/valve plumbed into the first for aux1 or aux 2.
My crude stick figure sketch of how I'd do it:
View attachment 83174
Having a sender for the gauge wouldn't be quite as easy, but could be done using the same switches and some relays. Being carbed, I probably wouldn't bother. Start out on the aux tanks, the engine will let you know when they're getting empty, then switch to the main and start watching the gauge.
Most of the posts I've come across about dual tank trucks is from people having issues with the reservior, so I wouldn't mind getting away from it. I also don't care for the potential failure point of the inline, frame mounted, high pressure pump. If I ever need to rebuild the fuel system in my dual tank Ranger I'll be ditching the reservior and inline high pressure pump. I'll go to high pressure pumps in each tank, the valve that Bobby posted, and a cartridge style fuel filter.