4bbl carb and has smaller gear reduction starter.. Has no provision for bracket support and none is shown w/mercury parts catalog.

This sounds like the same Delco "metric" gear-reduction starter mine has installed. If it's the same as mine, they're good starters but they do use special bolts with stepped knurling (steps from 3/8" to 10mm half way up the knurling). Not sure if your going that way but heads-up if you decide to repair block and replace the bolts, the bolts may be special.

Another thought, if one of the bolts was replaced somewhere down the line and it's not thick enough at the knurled area, it would place virtually all the shear load on the other bolt. This would be a quick and easy thing to check assuming the eng is already out.