Removing exhaust manifold

I have a log type exhaust manifold on a mid 70's 302 Indmar powered 1965 CCSN that did not survive the winter that I will have to remove and have repaired. It is attached with star headed bolts. I have three brief questions: 1. Is there a reasonably painless way to avoid breaking the bolts off in the block? 2. Is there a preferred method for the repair? 3. Is there a vendor for a replacement exhaust manifold? The previous fix looks as though it was puttied, brazed, or welded and then separated at the margin with the untouched metal.

Thanks Steve, The fix ended up much easier than we expected. I was able to find a specialty wrench to fit the spline bolts and luckily they were not torqued more than about 20 lbs and came out easily. The older log type exhaust was fixed at a local machine shop which determined that the middle two exhaust ports were raised somewhat from the outer ports and probably stressed the center of the manifold when bolted down. We had them shaved down in alignment and everything went back up great. One mistake I made was not checking the drain hole in the exhaust manifold for a carbon plug when I removed the drain plug to winterize. Thanks for the response hopefully I will be smarter at the end of this season.