[chris vickery]

Richard
The adapter plate is just that, an adapter. Don't get confused.
It was meant to use a 2 bolt crab type on a 3 bolt divers helmet type plate.
The issue here is that you now have a two hole plate which is correct for the crab distributor.
One time I changed out mine and it was a simple swap. I am somewhat confused as to why it doesn't mate. My suggestion of cam difference has been confirmed by the previous post. I was hoping that you have the correct length cam. I know that Mercury parts are somewhat different so there is a distinct possibility...
Alternately, a simple spacer may get you where you need to be if cam length is the issue....as long as the drive engages the cam you're good to go.