You have a bunch of options depending on your taste and budget. A number of vendors sell aluminum or stainless steel tanks that are designed to fit between the frame rails. The link below is to board vendor Dave Boyd's page. He makes a tank that will fit in our trucks with a bit of modification to the rear crossmembers, or he can custom build a tank that will fit exactly how you want it. I have one of his tanks and it's really nice.
http://www.boydwelding.com/category/...ts/67-72-truck
Here is a link to the FAQ section on how to install a Camaro tank.
Camaro Tank
http://67-72chevytrucks.com/vboard/s...d.php?t=448386
A lot of guys also run blazer tanks as shown below. A lot of vendors sell kits with these tanks too. They are also pretty available at junkyards.
Blazer tanks.
http://67-72chevytrucks.com/vboard/s...d.php?t=234732
I hope this gets you off on the right foot!