Exhaust riser bolts sheered

Hi, just had to remove the riser part of my exhaust manifold and as expected the two longer bolts sheered at the manifold end.
No big problem as have cobolt bits and ta thougp's etc, just want to know how deep I can drill out the old bolts, if I go too deep dose this cause a problem between water jacket and exhaust chamber?
My thoughts are to put two studs in this time with nuts on the top, I know it will be a pain to get it off over the studs but rather that than risk studs breaking next time.

Am I thinking rite?
cheers
Dave.
ps. season's underway over here and had a couple of good weekends in the warm lake already.

Since the manifolds are not handed meaning that there no left or right side ones all bolt holes are the same depth. Measure what you have, be careful and good luck. Or you see if you could find someone with a EDM machine to take them out.

Thanks guy's, what I really need to know is how far can I go without any disasterous outcomes, for instance if I drill through into the exhaust chamber will I be ok to put the stud right trough and seal threads with locktite or something similar, or will it go through the water jacket first?
The problem is that an inch is not deep enough to tap a new thread when you have a blind end.
probably easier to take it to an engineering shop but I enjoy doing this stuff.

last week we had a leak through one of the blanking plugs on our O M C 350 chevy manifold, (its in a 1978 nautique), it had rusted trough the inner end, unable to remove it so as the plug was mild steel we just mig welded it shut, worked a treat.

Mine are OSCO and I tapped them to get longer bolt in instead of cutting the bolt.