is that the 22g or a larger sd tank?
I prefer plastic too, but I found these in my "should be close enough to make it work" search
http://shop.broncograveyard.com/305-Gallon-Steel-Fuel-Tank/productinfo/602120/
http://shop.broncograveyard.com/305-Gallon-Steel-Fuel-Tank/productinfo/602123/
30 beats 20 and should tuck up in there better than the 40g sd
another Idea I've kicked around is adding passenger side tank with a duralift and toggle switch to refill while going down the road.
That is the stock 22 gallon tank that comes with trucks w E4OD transmissions...guessing they are thirstier????
I will stick w stock tanks...might toss 33 gal bronco tank in rear or build a 40 gallon rear..have drawings for 40 gal tank