I've been asked to make a tough petrol tank for a 4x4 for light off roading after the owner split the under body tank twice. I was going to fabricate one from 2mm sheet but thinking about it is there any reason an ex LPG tank could'nt be used? It's got to be 2-3mm thick, it's round so that adds more strength and it's got a sender unit already fitted. Can any body see problems with replacing one flamable liquid with another? Obviously it will be flushed out before the TIG goes any where near it for the feed,vent and return.

Good idea to my mind, beats old beer barrels!

Weight an LPG tank is very heavy and generally they mount by being strapped to frame.
The vent can be added by drilling the tank unit.
Adding a spigot fill line will be the biggest problem.

It's going on a 1800kg 4x4 with a rollcage, winch and a 19 stone driver not a 400kg single seater, I don't think weight will be an issue. The best part about having a light engineering/fabrication workshop is that making holes in things and welding them back up is not really a problem.

Can't think of any reasons why it shouldn't work. If you can pick one up cheaper than fabricating it then it makes sense.

why not just put a 'sump guard' under a standard tank?

It's not the ground that is putting holes in the tank it's things like farm jacks,shovels and sand ladders. I suggested putting the tank in the boot but it still needs to be able to carry the kids buggy and a weeks shopping.

That will work. Just make sure to replace all the rubber components in all the valves with petrol compatible ones. Spigot filler through the level sensor hole, take-off through the dip tube underneath the LPG shut-off valve (which is NOT petrol compatible!!!), block other holes, done!

Test req. for an LPG tank is 67,5 Bar burst pressure.

A multivalve (one-hole) tank woudl work as well, but is more work.