Sorry I haven't helped on this thread any. Been a busy week.
The big mouth, small mouth is for automatic (small) or manual (big.)
I got my big mouth nose cone from NAPA. Joe was correct on the price being around $15 or so.
All of the above is for a direct drive 27MT.
All of the redux drive 28MT units say no shim. Just some of the newer direct drive 27MT starters say no shim.
You are supposed to bolt the starter on without the solenoid attached. Use your hand to engage the Bendix gear by pulling the plunger back and then get your clearance measurement on how close the Bendix and flywheel teeth are. It takes more time, but it sure beats laying there waiting for a piece of metal to stick in your eye while soldier B hits the starter switch up top.
If you have a direct drive unit, where are you getting the rear brace from? They were discontinued by GM a few years ago is why I ask. Also, be sure and use new GM starter bolts. Yes, it does matter and there is a difference. Amazon has them for pretty cheap if you don't have a dealer close by.