We put a chamfer on both sides of the hole. The insides of these manifolds arent the smoothest anyway, especially down where the four tubes join......looks like a beginning welder. We ground the outside smooth but didnt try to do too much to the inside since we were removing the tube and improving things anyway. Then, the manifolds were jet hot coated and you couldnt tell where the weld was.

By the way, the flange studs on ours were metric thread. Was real confusing to the muffler shop guy. We finally used stainless
socket head bolts that work fine from the underside.

mike in tucson