Ken-
I just installed my Hermco twin bracket on my 79 23CC.
I hung my 30" Yamaha single this past week.
I followed Dons instructions to a T.
Pretty sure the bottom of the bracket is 3" away from the keel, and the top of the bracket is 26" from the bottom of the boat.
The mounting pad for the motor is an easy 6-8" higher than the bracket.
Don recommended 2nd or 3rd mounting hole.
I went with 2nd from the top.
Ill be spinning a 19" Rev4 (same prop from pre-bracket).
I have yet to float my boat (due to the current 20* temps in CT).
I can take any measurements you need while the boat is still in my shop.
Cory@ASMWCT.com