New tank, you don't weld them for obvious reasons :mil88:
Not hard to fit if you know what you're doing, if you don't have the foggiest about the gas system, entrust a LPGA qualified person to do it. Either way you need an LPGA qualified person, as the system has to be safety approved, else you wont get insurance on your vehicle and it is also a criminal offense to drive it on public roads on LPG (or with LPG in the tanks) if the certificate either never existed or is invalid (one way of invalidating the certificate is by a non qualified person carrying out work on the system i.e. fitting a new tank without having the work checked).
Don't just slack it off, if the police catch you with a tank like that, Fines, Points on license, and more importantly you will lose your car as it would be deemed dangerous for use on the public road. Don't forget, an in some respects an LPG converted car is basically one big bomb, if it's neglected and left with a damaged tank say, its a big bomb waiting to blow, it's only a matter of time till something will go wrong.