You could use a Td5 plastic tank, although it would need a /bit/ of fiddling to get it to fit.

When I built my 110, it had the brackets for a metal tank, and I fitted the plastic into it. You'll need to chop off the bracket on the crossmember (that the A frame bolts to), it's the one that one end of the tank locates on to.

Then on the rear cross member, you'll need to drill two large (enough to fit a 13mm socket and extension into) holes, about 3/4" diameter, on the outer face, then on the inner face you'll need a smaller hole for the studs on the tank support cradle to fit into. Trouble is, a non Td5 crossmember isn't flat on the inside, so you'll have to weld on a piece of thin plate to give the cradle something to butt up against.

On the A frame crossmember, the one you just cut the bracket off of, you need to fit a couple of M8 nutserts for the "other" end of the tank cradle.

Once you've done that, and offered up the tank, you need a couple of brackets for the top strap to bolt onto, this stops the tank from bouncing up and down (funnily enough, Td5/puma 90's don't have this, and the tank more or less touches the bottom of the rear tub, but not so on 110's)

I've got a load of pictures here from when I built the vehicle, I can email them over to you if you want.

On the top of the tank you'll remove the electric pump, and you can get a late 300Tdi pickup pipe to fit the plastic tank (as the last of the 300's, and the RoW Td5-spec 300's had them)

All in all, quote a faff about. OK if you had the vehicle stripped down in the first place, but a PITA to just "do" for the sake of it I reckon. better to get allisport to TIG up a HD ali one.