Dual fuel tank not switching when i hit switch

Hey guys, so my 83 k10 is a dual tank and that is partially why i fell in love with it, but alil while back my selector quit selecting. And it isnt the switch up in the cab. It has an inline fuel pump in it plumbed in right where it looks like mayb the selector is rightoutof the tanks? But i see the two vent lines that are smaller, they tie together from both tanks, and goup to the front.thr other four lines come from gastanks and pipe into what i think is the selector, both tanks go into this unit top ajd bottom lines, then they come out of the unit with single lines,.which are goin back to engine, the one side goes into the inline fuel pump. The other goes upfront with nothing else tied on. My question is if i were to cut into the system and install two shuttoffs with a bypass in between so i could manually switch tanks if i wanted to, would i do so right after the unit and before the fuel pump?or would i have to do.it where there are the two lines comin from tanks before they tie into the unit with wires on it. I will take pictures today to show what im tryin to say. Besides that unit lookjng expensiv, id hate to be out hunting and nit be able to access my other fuel tank.which is why id like to have a manual valve setup. But im jus not sure how to go about it, so that the gauges will still read the level of fuel. Thanks sorry for th e long ass post

To me that would be like taking a giant leap backwards.
But here is info on the switching valves-