I user 3/8-16 black oxide carbon steel allen set screws at 1 1/4" long with AN series washers (not lock washers) and stainless nuts. All from McMaster-Carr
www.mcmaster.com for about $27 and I then have 84 extra nuts and washers. I also use the 3" bolt spacing which allows a lot of room around the tube. This may not work if you use the 2" spacing.
I put anti-seize on the set screws and back them off about 1/8 turn from bottom. I also run copper header gaskets that don't compress and blow out.
Once tightened after the 2nd or 3rd heat cycle they stay tight.
Jim