The first problem I had was chunks of weld around the inside area of the headers and side pipes so they would leak and blow out gaskets. When I put a straight edge on either there were high spots. I fixed that and got new gaskets.
I used regular grade 8 bolts and these if I remember right.
but might have been these.
With the flat matching surfaces they never came loose. The bolts are used once and typically break when you take them off.
Never reuse the bolts even if they come off.