Chris and KlausC -- thanks for the continued interest in my "problem".
Like most combination wrenches the box end is (was) slightly angled to the handle part. By "offset" I am referring to the type box wrench that has the boxed ends parallel to the handle part but has a 1/2 inch or so drop from handle to box. That amount gives me clearance to pass the "shelf".
Once the bolt is cracked loose I will not be able to unscrew it all the way due to the vertical stud--but the 2 remaining bolts are accessible by the 1/2 drive breaker bar and Snap-on impact socket which I used to loosen the first bolt. Once those are removed I'll be able to back out the problem bolt by pulling the cast bracket away from the block in stages thus allowing the bolt to unscrew all the way--it will still be trapped in the bracket by the stud but free from the block.
I would think it possible there may have been a scratch or nick in the broken wrench--but keep in mind I was applying a lot more pressure than it was designed for. There is just enough room to put a short 3/8 drive socket on the bolt head--but no room to attach a ratchet, breaker bar or extension. thanks again. The saga will continue once home next week. --- Dave