I used the kit like the one from LMC got it from classic trucks, it is out of a blazer, here are some links to how we installed it and how far it hangs down.
1. We originally thought we would go with the filler neck as it comes (top left angle fill)
http://kieth.smugmug.com/Trucks/66-G...005429_gWBxmmr
2. we also wanted it as high as possible so we modified the bed crossmember by installing a piece of flat bar after we removed the center of the crossmember so the bed bolts would not hit the top of the tank.
http://kieth.smugmug.com/Trucks/66-G...005088_DrSmCmf
3. As we progressed thru the build we found that we wanted to bolt the tank to the bottom of the frame and make it easy to drop if we needed to remove it, so we welded nuts to the frame flange to make this easier.
http://kieth.smugmug.com/Trucks/66-G...478344_sr9cCz6
4. this is how it looks bolted in without the tank straps
http://kieth.smugmug.com/Trucks/66-G...997329_svjGkRJ
5. As we installed the bed wood we realized that the side fill part of the tank was not going to have enough drop to facilitate filling the tank, so we decided on a different approach. We cut the fuel tank filler neck off and added a plug and brazed it so you could not tell it had been there, and cut a new filler neck hole that was a straight drop through the bed wood.
http://kieth.smugmug.com/Trucks/66-G...270029_44JXfxh
6. Here is the finished product:
http://kieth.smugmug.com/Trucks/66-G...204975_sMtpxWj
7. Mystery---how far does the tank hang down ? well I have not gotten the rear bumper on yet to figure that out, plus I run rear air bags so I can adjust the ride height.
the tank hangs down 7" from the bottom of the frame (about halfway covers the 12 bolt rearend center cover........Kieth