Hmmm--sounds familiar!..
When I bought my 82 K20 Stepside with a 6.2 a few years ago,its dual tanks were all fubared too! --the previous owner(who might have owned YOUR truck,by the sound of your post!

) had chopped out the dash switch,and the one that controls the tank switch-over valve under the cab,and the valve itself--and added manual a shut off valve instead..all the wiring was snipped off with cutters,taped up,and all the hoses were cracked and brittle!..its no wonder they couldn't get the motor they put in started!...it was a plumbers nightmare under the truck where the hoses run!..
Since the drivers side tank leaked,and its sending unit didn't work,I decided to just use the passenger side tank only--I can't afford to fill both tanks anyway,and it would take me months to use all that fuel--I rarely go very far with the truck,and one tank will go for 400+ miles anyway!--so rather than hunt all over hell to find a used dual tank switch setup,and 100 bux for a new tank,I just re-plumbed the one "good"tank,and called it "good enough"...
I spent at least a week playing with wires,and looking at wiring diagrams,and decided it was not worth the time or expense,just to have 2 tanks I couldn't afford to fill !..so I got the gauge to work on the one "good" tank,and that's as far as I'm going with it!..
I suppose you could find replacement switches,valve,and make it all work again--but is it really worth it??..
