I plan on putting in a 31 gallon square-body suburban tank in my 70 (LWB). They also make one that is 40ish gallons. The standard square-body Blazer/Suburban tank is 25 gallons if yours is lower a good bit. You will have to make a custom filler for the tank. Either behind taillights, sidemaker or inside bed corner.
Here is one of the videos I was referencing.
https://www.youtube.com/watch?v=2e-J_0_djjE
It is for 4x4, but should work on the 2WD model
Here is another from an older board member
https://67-72chevytrucks.com/vboard/...d.php?t=218898
The attachment is another one I save from somewhere many years ago.