Kern, are you planning to do a continuous spot weld technique with a bit higher heat setting to weld up your but joints?
I may try these or make my own version of them. This with my chop saw for straight cuts looks like it should work. Spot weld them in place and then remove to fully weld.
https://www.etsy.com/market/welding_jigs
Some exhaust tubing at reasonable prices here, but it's high quality stainless. We can get away with mild steel here in California from Summit. don't forget to drill weep holes at the bottom of the pipes in a few places to drain condensation.
Adding an ovalized cone instead of a factory style exhaust tip really improves the exhaust note, much like the flared ends of musical instruments. It somehow sweetens the sound to the human ear. Those ancients knew a thing or two.