Water pump bolts

The current alt bracket is a home fabrication type job, done before I owned it. it only uses the top water pump bolt. (The one circled)

I have since got ahold of a proper small block Alt bracket.

It was during the removal of the lower waterjacket type bolt to fit the new.alt bracket I stumbled upon the problem.

Now I'm wondering if this is the reason why the old bracket wasn't refitted. As this bolt may have been an issue? wrong size? Wrong width? Shredded thread inside the block? Who knows

Hence me asking if anyone had the correct sizes so I can get one and try that rather then buying a complete set.