Don't have it figured out yet. Mike your issue sounds different though. My issue is with a small block.
The service bulletin only shows auto applications. Is the 8420 the 153 tooth option for manual?

 

I found this on another site that might be some help.

Starter for 4 speed with 621 bell [Archive] - Team Camaro Tech

 

There's a manual trans omission on the previous chart because that TSB was referring to having heat problems with automatic trans applications. Must have been due to the extra heat that's generated by automatic transmissions.
The #8420 starter was the replacement for the #8361, which was used for both automatic and manual transmissions. See chart below.
You didn't show a picture of the starter you're trying to use, but for your 153 tooth (12-3/4") flywheel, you need to be using the straight across mounting hole configuration on the nose, not the angled mount.

 

Bellhousing is a 3858403.

 

That's the right one for a 10.5" clutch assembly. like you have. That model is the vanilla flavor of bellhousings, used with and compatible with all 4 speeds of your car's era, using a 153 tooth flywheel.

 

And I don't know why you're barking up the bellhousing tree? As I stated before, according to the #1108338 starter you say you're using, it's the WRONG ONE. The angle mount starter gets kicked further away from the block (towards the passenger side) to mate up to the larger diameter 168 tooth flywheel. It will not clear the bump-out opening on your 153 tooth bellhousing. Even if it did, the starter teeth wouldn't mesh with your flywheel teeth. You need the straight across mount starter.

 

I now have a z-28 starter in hand. Have not been able to try it yet. The bell was the only model used on z cars, so I read. The block was used on z cars too. I had returned the other starters right away, so I have not had the 8338 in hand for a while.

What has happened with the other units it that the nose cone hits/bottoms out on the bell before the mounting holes are aligned. If that happens again with this unit, I may have to assume the block was used on an application where only 168 tooth and larger bells were used. Seems like a reasonable theory, though it makes no sense when thinking of how blocks were used/shared during production. Can't imagine unique starter locations for a block that was used in multiple cars/trucks. Further, the only applications in 69 were passenger cars, as far as I see.