The gas tank, to do it properly, needs to be emptied, cleaned by several
rinses and then inerted with gas while it is being welded. Your welder will
have a procedure for this.. His inerting gas must be what ever is suitable
for aluminum welding (mig or tig).

Cleaning and gas freeing the tank is a small problem. It would be the gas
freeing of the bilges that concerns me since it sounds like you have to do
some cutting to get into them and the tank.

You ask about builders responsibility.. I would say yes, but you would have
a hard time getting him to do anything if you can't take it to his
facility... He's not going to want to pay for the gas free and welders
services in someone elses boat yard..

In the typical US boat yard, your talking about several thousand even if
everything were simple.. It cost money to have a job like this done
correctly and safely..

Steve
s/v Good Intentions