Many of the 70's Ford aft-axle tanks were actually held in place by a skid plate with the top of the tank pressed against the straps between crossmembers. You could also build such a skid plate for the Blazer tank. Both of those solutions will give you 30+ gallons and not hang down too far for your offroading. For even more capacity, I'd suggest a 2009+ Dodge 4500/5500 aft axle tank at 50+ gallons. Same method of attachment and I haven't yet poked a hole in mine at work - although not for lack of trying.
All of these fill at the top of the tank from the driver's side fender area.