Exhaust leak and manifold bolts.

[AirSpeed]

Recently installed a new 350 in my 72 gmc, been about four months and last week I noticed an exhaust leak on #2 cylinder. I tightened up the last bolt on the manifold and it went away, today it came back. Can't remember what gaskets I used, I did buy the "better" set from napa but I'm thinking it's probably burned through. I used the old exhaust bolts and after cranking on that sucker I'm afraid to tighten anymore, I'm afraid it will break off in the head. I plan on grabbing a new set of gaskets and replacing the bolts at the same time, do any of you know what length bolts are used in the exhaust manifold? I don't want to pull them until I have the new ones. What grade should I get, do I go with grade five, grade eight? . Anyone have a suggestion on good exhaust gaskets? Also am I doing any damage running with an slight exhaust leak? I won't have time to fix it until this weekend and I need to make a 70 mile trip tomorrow morning.
Thanks for any suggestions on gaskets and bolts and any other reason I am getting this leak!