I did that on my 87. I went with a poly tank from Summit. Didn't want to risk sparks in case something pierced the tank. It's a fairly easy conversion.
CaptKaos used a Blazer tank, here's his article;
http://www.captkaoscustoms.com/blazertank.html
I followed in the steps of bigblock73;
http://www.73-87.com/7387garage/chas...ellinstall.htm
Only difference is I used square tube instead of c-channel for the crossmember.
I don't have my finished pictures here at work. I bought a 6" deck plate from a marine store to use as access in the bed floor to the gas cap. I did run into a few problems along the way, had to tweak the placement of the fuel pump & I still can't get my gas gauge to work.
I don't know the 67-72 frame well enough to say what differences there are in your mod.