I thinking of maybe something like one of these tanks but assume that the tank would not sit up under the cab at all and that would be issue for connecting the filler neck, may give this a shot and see how it measures out
https://www.classicparts.com/1982-86...tinfo/23-952/#
with the associated brackets
https://www.classicparts.com/1973-91...ctinfo/23-989/
https://www.classicparts.com/1973-91...ctinfo/23-988/