Whoa! Whoa!...I did not say it has little stress
I said it has the least amount
...and I was also figuring it was a mast head
rig..if your fractional , and it sounds now like you are from what you said then the stresses go up quite a bit.
And yes that would be TIG or TGAW needed to do the job correctly. A good welder will still need to do this from the outside though and not so much if any from the inside as you keep saying....TIG is not a tight confines way of welding...by humans anyway...and he will drill a stop hole as has been suggested and will also grind a V and run a root pass. Possibly fish
plate it as well.
It will be 50 feet in the air.... at this point you want it repaired and strong not necessarily looking like new at ground level...
Piece of cake for a qualified welder.