Yeah you can bolt in a 71-72 blazer tank, they come in 2? different sizes and are available through any resto catalog for around $200 for a tank,sender and mount straps or through the vendors marketplace here by brothers trucks and others. If you use a blazer tank you will have to fab up a filler neck, most use some exhaust pipe and a few pieces of gas filler hose to get the filler cap where you want it, some in the bed floor, through a stake hole, on the inner bedside, or the outer bedside with a fuel door (puke). The other downside of the blazer tank is you have to cut out one of the bedsupports on the under side of the bed and I believe you need to trim one of the frame cross supports.
Another option is the first gen (67-69) camaro tank. you'll have to do a little fabricating there too but the filler neck will be behind the license plate so other than buying a flip down tag bracket at pull a part theres no mocking up a filler.
heres a link to the camaro tank how to
http://67-72chevytrucks.com/vboard/s...d.php?t=448431
I havent found the blazer tank install how to yet. If you look at the very bottom of the page when you click on forums, there is a forum category called "FAQ" frequently asked questions, there is a lot of cool how to threads in there.