intake, weld or glue
If tig welding area I can get to, how to seal off the area's I can't get to, with something like JB weld or some structural adhesive?
Instead of trying to weld in areas that you can`t get to, change your plan, man.
Cut tube material flush with flange, run tube through holes and weld from the inside.
I`ve built my own custom ss headers that had to turn tight immediately out of the flanges.
All the turns were made with prebent 90-45* ells with segments out of them.
I tacked each of the 4 runs complete for mockup assembled, once proven, each were removed/welded/reassembled. Turned out not too shabby for a 1st time header build.
It helped allot that I`m in the pipe fitting trade, but headers are a whole nother animal.
Same tactics could apply for what you trying to achieve. good luck, show a pic when done.
