Parts and stuff you need:
3/64 in.(give or take) sheet Lexan or similar sheet plastic(Plano parts box type plastic works best
3 ea. 4-40 X 1/4 flathead screw
3 ea. 4-40 X 1/4 buttonhead screw
3 ea. Associated 4-40 small plain nuts
6 ea. shock O-rings
1/8 drill bit
1/4 drill bit
Sharpie permanent marker
1. Cut out a 2-1/4 X 1-3/4 piece of Lexan/sheet plastic.
2. Carefully place the plastic plate under the front of the fuel tank and mark the three screw holes with the Sharpie. Drill out the marks with the 1/8 drill bit.
3. Place the plastic plate on the fuel tank area of the chassis BUT reverse the mounting holes in relation to the existing tank mounting holes in the chassis(i.e. one hole in front, two in the rear). Try to center the holes the best you can. Mark the new holes to the chassis and drill the marks on the chassis with a 1/8 drill bit. Countersink the new holes on the underside of the chassis.
4. Temporaily mount the plastic plate on the chassis' new holes using the flathead screws and plain nuts and mark the plate with the sharpie to indicate the front. Mark the old tank mount holes on the plastic plate from the underside of the chassis using the Sharpie. Unmount the plastic plate and drill out marks with 1/8 drill bit.
5. Drill out the old tank mounting holes with the 1/4 drill bit. These will now be the access holes to access the screw holes in the plastic plate mount.
6. Mount the plastic plate to the chassis(notice which way faces front) to the new holes using the 4-40 X 1/4 flathead screws, plain nuts and 3 O-rings. Sandwich the O-rings as you would when mounting the tank. Use Loctite on the plain nuts.
7. From the underside of the chassis, install the three 4-40 X 1/4 buttonhead through the new access holes in the chassis and into the new tank mount holes in the plastic plate. Install the O-rings and mount the tank to the plastic plate.
There you have it!
This is essentially a tank soft mounted to a plate which is in turn semi-soft mounted to the chassis.
This "Fix" reduces the amount of vibration transfered to the fuel tank and the new mount "gives" in the event of a driver's side T-bone hit therefore reducing the chance of a broken or "split" tank.
You can play with the "softness" of the plastic plate by drilling holes or trimming it in strategic areas and/or using different thicknesses of plastic sheet.
You also can use more O-rings to soften the mount or to raise the front of the tank.
NOTICE: This may not eliminate the "leaning out" condition when tank is 1/8-1/4 full.
This "Fix" works with the stock chassis. I don't know if it works on some aftermarket chassis because some have lightening holes under the fuel tank area.
Hope this "Fix" works for you!