yes.. it will fit in the frame rails.. you might have to cut the floor out and have it flush with the bed floor for clearance issues.. or get a chevy blazer gas tank.. it will fit in there for sure.
if you do that you need to put it half in half out (the mounting holes on the sides of the tank bolted to the bedfloor) because behind the wheels you have to worry about dragging there too when you lift the front dump the back for dragging and whatnot. Honestly the best bet is a fuel cell up against the cab at the front of the bed.