i know on the the older international trucks (my 1968 is like this) the tank was under the bed/cab on the passenger side and the filler neck came out of the cab but it was down low not up where it is on these old Chevs and yes in 73 chevy took the tank out of the cab and mounted them on the outside of the frame on both sides (if dual tanks) they called them saddle tanks in 88 with the new styling the moved the tank inside the frame rails funny think is in 73-87 the tanks were 16 (short box) or 20 gallons (long box) on my dads 88 chevy long box 1/2 ton the tank is 30-35 gallons the tank inside the frame rails is good idea but makes it hard to route exhaust so i'd just keep it in the cab best place for it, lots of protection and will not be near any hot exhuast pipes
