I was able to use a YJ 20 gallon plastic tank in my friends' '86 Montero. The old sheet metal tank was down to about 11 gallons after getting smashed numerous times!

I made my own skid plate/carrier in the style of the YJ original, but out of 1/4" plate on the bottom and 1/8" sides. I had to make new mounting tabs for the crossmember behind the rear axle, I cut the old ones off. A ZJ sender unit on the YJ in-tank module with a resistor added works the stock fuel gauge with good accuracy. I had to make some adapters for the fuel filler hoses, but it wasn't too hard to do. I re-routed the tailpipe to come out the right side between the spring and frame, instead of straight out the rear. It's been in there 10 years or so, now.

If I can remember, I'll take some pics of it the next time he brings the truck by my house.

P.S. If I had to do it again, I would build a full custom tank, and get the capacity at about 25 gallons by eliminating the angled front corners of the YJ tank and making it conform closer to the contours of the floor and frame.