'fuel capacity of '73 long bed 3/4 ton 4x4

[dudevato]

I just got this truck, a GMC with a 350 with a 3 speed plus granny low. I have a filler on each side of the fleet side type 8' bed and all looks to be factory under there,

Can someone tell me the tank capacity of this setup?

Also is the electric switch switching system prone to problems? The gas gauge does not work on either tank and the whole system seems overly complicated.

On the 200 mile drive home I stopped to fill up and the right tank spilled out fuel when I took the cap off. I put it back on and filled up the left tank,

I'm already thinking of a brass valve coming up out of a hole in the floor and a 'left-right' switch to feed the gauge.

Any and all info on the subject is gratefully accepted, thank you