When I originally had the tank welded I was told the tank had to be new , the left over oil residue in a used tank from pre mix will mess up the plastic welding process.
I was worried about welding at first as well. The guy that did the work has owned his own plastics company for over 25 years.
He had done 2 other tanks for a guy in town before mine. Both of these tanks have ran the past two seasons with no issues, and both bikes are being raced. Both are two stroke CRs
The tank was also tested with pre-mix before it left his shop for leaks and held for two weeks, but still has not run next to my hot cylinder head yet.
I've also thought about running one of those pro circuit heat shields like the four strokes use, to help to cut down on additional heat and help to hide the welds.