I buy pre-bent mandrel tubing. The additional costs are for clamps, fixtures, jigs, expanders, bandsaw fixtures, etc to be able to make a reproducible product. Once I make one, I can duplicate it without the car. It is a one time buy-in which typically pays for itself.

After doing some math, I am leaning towards the 1 5/8" tubing. It is right in that sweet spot of what I need currently, but it isn't without it's limits.

I've ordered 2 sets of exhaust header flanges, and 2 sets of merge collectors. These first sets will be mild steel, until I can get some flanges cut from stainless.