Guys good thread.
The old school Mallory YL unit is the smallest out there and they are fairly cheap cores ( everyone wants a YL), can be machined for the flathead , give a good advance curve when set up and run great.
Thats a YL in the center. ( red or blue cap) The one on the left is the YL Marine unit using a crab cap for low clearance in a marine app like a boat with engine cover . Also looks pretty cool in a flathead...