So here is probabably the best shot I have of the gas tank:
What I did was first move the rear most cross member back about an inch, then cut/grinded on the frame to clear the tank.
Then I took some 2" wide aluminum strips and bent them to the contour of the tank. I bolted the one end to the rear cross member, then used long bolts on the next crossmember to draw the tank up and tight against the frame. I kept some of the foam padding that was originally on the take as cushioning too.
The filler neck sits right in the frame rail, so I use a hole saw (2" I believe) and cut a hole for the rubber filler tube to pass through. I'll have to get a picture of this. Since I just got the bed on, I haven't finished where installing the filler tube. I'm actually not 100% sure where it's going to go.
It really wasn't too hard to install the tank, once I had the tank in hand.
Good luck.
Oh, I got a little bit of work done yesterday. I began modifying my 70 GMC core support to be like the 68 chevy core support. I made a little pattern with some paper and pencil (rubbing) then drilled holes where the headlight adjusters pass though. Then I started cutting the corners of the holes to make them square with my dremel, but the battery died, so I'm not finished. I'll have to get pictures when I finish.