Of all the bolts I tried, the 5-40 felt best so I took a dare:
I tried a 5-40 Starting tap by finger, and it felt right so I went slowly in and out to chase the first two or three threads.... then tried my bolt again and she screwed in to the full length easy but snug
Then I took the tongue muffler to install, and noted the muffler holes were ever so slightly too widely spaced but real close
Assumed some previous attempt to install the muffler caused the engine flange threads to get buggerd a bit
Since the muffler holes are un-threaded I just ran a next step larger drill through one of them to gain a schoche of clearance and slop... done and done...