I use Bill Edwards @ Woolf Aircraft. You can get as technical as you want. I usually start with their header builder kit. You can determine what diameters you require, then add on from there should you decide you want 100% proper nesting etc. Their header build kit, I believe comes with a set of merge collectors already, sized to your specs. You need to decide if you're building a stepped header or not before hand, then order additional sized tubing accordingly. Woolf only uses top quality USA/German made SS material. Last kit I bought came with waterjet flanges, which work just fine. Should you want cnc cut headers, you made need to check other sources, but ask Bill. IMO, a waterjet/laser cut flange is just fine. You made need to do a little bit of deburring, but thats no big deal imo. WJ, lazer or cnc flanges will all work fine, just depends on your budget. Here's a set I build for a buddies mini cooper using a Woolf kit from Bill. Edit: Pretty car! Edit edit......3/8" thick flanges minimum