[Dominik]

If mounting clearance is no problem I suggest using a stud kit with the serrated washer/nut. (Don't know the name). You tighten them with your finger and need a spanner (7/16) to release them!

If your clearance is critical (for mounting and head dimension of the bolt), I use hex head ("Allen") bolts with reduced head.

3/8" flange is a good choice, however do individual headers need that?

I used sealer, no gaskets.

Lastly, I had a bit of misalignment from the 4-into-1 collector slip in. So I left 3 out of 4 headers slightly loose, slipped the collector on and then tightened the header flanges. Never gave me any problem.

I do not recommend the flange mount (headers to sidepipe), in fact strongly advise against it in spite of 100s of cars using them. Slip-in is the only correct way.