225/slant 6 exhaust bolts broken

Unfortunately, people ask >$100 for those exhaust manifolds. If you luck upon a slant in the junkyard, maybe $20. Might be an opportune time to put on after-market headers if you were ever considering that, but that takes time.

You will still need to remove the broken off studs. StrokerScamp mentions welding a piece to the end of the stud to allow removal. That is one way, but I have never tried. I have always drilled a hole in the center of a bolt. Getting it centered is the tough part. Drilling will be much easier with the manifolds out of the way. I suggest drilling it with ever larger bits until not much is left, but don't get into the threads. Then use a tap to take out the rest of the stud. You can try a screw extractor, but they can break off in the hole and cause a bigger headache. If you try that, get the best quality ones you can and drill the biggest hole for one you can. When you install the new studs, use thread sealant since I recall those penetrate into the water jacket. I like sticky "gasket sealant".