I replace and rebuild Toyota 4x4 frames as well, and have replaced the frame from the middle of the cab back on my '84 XtraCab. Instead of one 2.25"x4.75" rectangular frame rail per side, i have two 2"x2" over-under frame rails per side. I also relocated my crossmembers, so my 30-gallon tank is not mounted exactly stock.
I could go look at an XtraCab i used to own, and build a larger-capacity tank that would mount to the stock crossmembers though.
My explanation of my frame may not be real clear, so here's a pic. The tanks are located on either side of the driveshaft, the exhaust runs between the driver's side frame rails. (Tanks and exhaust not installed in this pic. Picture was taken from inside the truck cab, after having cut out the back of the cab).
