Your experience serves you well. You can see in the picture below that the holes are well under an inch.



Here's the mount that's still in good shape. It almost looks like the bad mount had a top nut that's gone - there's no sign of threads at all.



I'll probably relocate the holes per Bob's advice and use either a threaded insert or a stud insert. The old hols will get a dab of RTV to seal. Will post when it's working. Thanks for the help.