Tubular header welding advise.

[AC sports]

In prep for the first race of the season and this happens under full power going up hill. Totally lost engine power. Was amazed how much difference it made.
Now how do I go about fixing them?
Mig or Tig?
Can I weld along the fracture line? Or will the metal be too brittle there?
I'll be welding in some straps, from flange to each individual pipe as a support also.

[cjperformance]

Depends on how well it fits back together and how neat you want it to look.
Basically you could just clean it all up inside and out. Bolt the header plate to a head.
Get the loose pipes tacked back into place.
( not needed but for a better finish, Plug the pipes and TEE off of your gas feed to allow your shielding gas to fill the pipes from the inside.)
Weld them up , the mig is quick, the tig is clean.
Before unbolting the flange get the oxy torch out and cherry red every pipe, this will do a backyard destress and they wont crack again.
(If you will add bracing , destress after bracing and before unbolting.)
Get the die grinder out and clean up and dags inside to pipes,
Go racing

[gunt]

The pipes we not happy in their previous forced location , i've see this a good bit , i seen the other advice , i wouldn't heat the pipes as i would weaken them , but i usually get a few new bits and bends cut back the existing to where it allows you to fit in the new bits to bring the alignment back to suit the flange , as the pipes are now happy where they are so leave them there and just make up the min difference

[rebelrouser]

when I build headers I mig weld the flanges, both sides, and then gas weld the tubes on the bends. The issue with mig welding headers is if you are not careful you will get wire and lumps on the inside of the tube. A lot of the old header makers recommended welding the flange on the head side, and using brass to support the tube on the outside. To fix those headers right you need to weld in new stubs and mate your existing tubes. A lot of work.