I'm with S10 on the ARP header bolts.
Get the stainless ones, they're stronger than grade 8's.

Far as header gaskets go, I've given up on aftermarket hot rod gaskets.
Some of em melt like Kleenex when they get wet.

Run stock gaskets.
Shim stock style or the aluminum sandwich/asbestos looking stuff.

Either one is better than the aftermarket stuff.


Far as thick header flanges go, 1/4" is sufficient.
All you need is to make sure they're flat.
I build my own headers from scratch and use 1/4" cold rolled for the flanges.
The tubes are TIG'd to the flanges on the inside and a bead of braze is run around the outside.
The braze reinforces the tubing/flange connection.

Header paint sticks just fine on braze as long as you get all the flux off.