I need to weld in a cat, i have no welder, but i can get a crappy non gas mig to make a jig. here is what i had in mind.
Bolt 2 3" flanges to each end and weld a pipe to connect the two pieces together.
then I can pull the downpipe out and cut an 7" section out, mock it up and bring it for a finish weld.
I know i can just use a marker and mark the pipe and just line it up but I dont want the downpipe to change in degree with the flange, if that makes sense.

Or if anyone here wants to chime in and help me out. thanks in advance.

 

You shouldn't need a jig for something this simple. Measure what you need to cut out of your tubing for the cat. Cut the tubing and weld it back in. At most you would just need to tack everything with the tubing bolted up under the car before welding it.

 

Just take it to a muffler shop or use the mig, doesn't matter if it has no gas , as long as it works

 

as long as your pipe to cat to pipe fitment and marks are good and strait there should be no problem.

 

the car itself is your jig. if you can attach the whole exhaust system together along with all the hangers and mount, should should be able to mark where you need to cut... cut with sawz all ... remount each end...tack in the cat with the new flange.. and you can be sure it'll come out straight since it did all the measurements and tacking while it's on the car.