TD5 tub onto 300TDi?

Hi folks, repairing my uncles 300tdi after an altercation with a collapsing tunnel. Both rear corners a squashed so a new tub is the logical way forward.
Spoken to a very helpful chap local (ish) to me who I've arranged to buy a replacement tub from. Only issue being his tub is from a TD5 and mines a TDi, filler caps in a different location.
Anyone got pics from how they've extended the filler neck to compensate for the difference?

Could move your fuel tank?

is that going to mean welding new mounts or changing the tank? I want to try and keep this as simple as possible!

After an afternoon of head scratching, a cutting disc and some tigerseal will be my best bet I think