BB exhaust studs

Local quality hardware store. 3/8-16 setscrews In whatever length you need. (You may have to go to a bolt specialty shop, or order from a place like McMaster Carr.)
Also, I would check the threads in your heads. A 906 head has all the exhaust manifold bolts into water. I don't know for sure with 915s, but I'll bet they are wet too.
There should be a lot more the 3/8 thread depth. I'm guessing you have some serious rust or thread sealer built up.