Header Fabrication

Some good tips from 900HP and Exhausted (Calvin Elston) if you skim through here 340 Chrysler Street Header Questions - Don Terrill’s Speed-Talk toward the bottom of the first page and then onto the later ones for what you are asking about blacksmithing.

Probably more in this thread - I can't recall for sure if the flange to pipe fitting was discussed.
First time header build - Don Terrill’s Speed-Talk

As long as the pipes off the flange are reasonably long enough, the log header can work very well.
See Calvin's posts about blowdown, and if you're a member, follow Hoffman's link to Clavin's thread about when he tested the theory using by making log header. If not a member, Hoffman quotes snips that cover the takeaways/
Novice exhaust question: What is "blowdown length"? - Page 2 - Don Terrill’s Speed-Talk

Another source of tips on welding up flanges and pipes is Headers by Ed. If you can get past his curmudgeoness, he built nice stuff. Not sure whats on his website but worth poking around.