Header qustions???

I'm sure you can mill the intake,,But,, I run a old set of Direct Connection headers 6 into 1 and all they have is 1/4'' nuts welded to the flange out of the box. Worked fine with my Offy intake. I also run 2 intake/exhaust gaskets to help sealing and to take up any imperfections. Been on the car 5 years now.