I'm in the process of cleaning out the tank in my 67 c50 right now. According to Wes at Classic Heartbeat Parts, the tanks will interchange with the smaller trucks. 6 cylinders have 1/4" line and 8 cylinder has 3/8". My sending unit float was rusted out so I ordered a new one and some other parts from him.
A pressure washer got most of the crud off the tank floor and walls. I plan on using some pea gravel and muriatic acid to knock out the rest then use KBS coatings 3 step tank sealer. I had the first 2 steps from a previous project so figured I'd try it out.
https://www.kbs-coatings.com/tank-sealers.html