I just was wrenching there earlier this year, but don't remember the size off top. But I use an open wrench that has a slight bend to it, a straight wrench is too tough to get a good bite on the nuts to really lock em down. I like the RJC milam gaskets, over copper gaskets.
Also, while off take a look at the header flange, mine had some valleys and slag. This probably explains why I was cutting through copper gaskets so quickly. I surfaced the flange with a belt sander and fine grit paper. Don't get too aggressive though, you don't wanna cut into the flange, just smooth and even the surface. While you're at it, it doesn't hurt to double up on the nuts. Nut to nut, I now have 6 on my 3 bolt.
And, hit the nuts again after letting the RJC milam gasket settle for bout 30 mins or so, since it'll wanna decompress some. After running the car around a few times, remember to check the nuts again to see if they've loosened up at all. Should be bullet proof at that point moving forward.