I had a 72 I did this too but I didn't use a blazer tank, seems like you have to move a crossmember to make it fit. on mine i just moved the crossmember and bought a harwood 15 gal fuel cell. then cut a hole in the bed for the filler
kinda nice in that you can see what's in your tank and not wonder how much gunk is sloshing around down there.
but then you have a hole in the bed. but if your building a prerunner it probably wouldn't matter
just usually a cheaper alternative to the blazer tank. which can be rather expensive.