8.8 sounds like a metric designation. get a couple of 3/8 16 nuts to check them. put whichever starter bolt thru the starter. measure what thread is left. screw bolts into holes in block. make sure they go in further than the amount that stuck out of starter.

AH, you are correct.... it seems that 8.8 is the metric equivalent of a grade 5 SAE bolt. Those clowns at the boat dealer don't know their business.... they've sold me the wrong stuff too many times.

According to the ATi tech rep. It is uncommon for teh small flywheel; however, not a non-possibility. According to a career boat mechanic friend of mine "mercruiser mixes/matches parts as they can get their hands on them from the OEM"

But the OEM never made an external balanced 153 tooth flywheel for either the 400 small block or the 454. So there were no parts to mix and match. 350 and smaller? Sometimes. And if it were, the 2 starter bolts would be side by side, 1 long and 1 short.

Ayuh,.... Many, but not all blocks are drilled, 'n tapped for both starters,....
'n of course, Many blocks out there were only drilled, 'n tapped for 1 starter, or the other,....
Ya won't know if yer block has all 3 holes, til ya look,....

As for the dowel,... Ya need whatever is a snug fit,....
If yer into the threads, a 3/8" dowel won't fit,...
The step in the block drillin' may well be 3/8",...

Ya gotta love the newer starter bolts with the longer 1/2" heads,...
More room for the socket to slide on,...
Worst bolt to get a 9/16" socket onto is the inner bolt on a straight across pattern starter,...
The flywheel cover dust shield is in the way,....

Double check if the inner hole near the flywheel is there and threaded-which it will be. Screw a bolt in there and be sure.

Well, I'm throwing in the towel. I was able to get the drill in there, but it is so tight I can't effectively use it. Based on the amount of cussing I've done and how sore my back is I think pulling the engine would be less miserable.

...but I may not have to do that, as the other bolt hole for the mini starter is threaded. I just have to find the right starter now. I examined the teeth on the starter gear of my old one, and they all have a bit of angle on them, so I think is jacked anyway.

I really thought I'd enjoy having a big block inboard... "it'll be much more fun to work on, I said", "A standard engine has to be way simpler than an outboard, I said"....

My hat is off to any boat mechanic that can go in every day with a smile on his face.

Would this guy fit the bill?
https://www.summitracing.com/parts/pwm-9100/

Examined teeth on starter gear of the old starter? They are supposed to have that angle cut on them. If they were cut square, they would hit flywheel teeth and stop engaging. Starter I showed earlier is the same, except for the price...

By angle, I mean the teeth look like they are folding over, like the veins of a water pump impeller.