Since you're planning on having it done, go see Hermco in Jensen Beach.
Here's his site:
http://www.hermco.net/
Don is also a moderator on this forum and you'll see he is not lacking in SeaCraft transom experience.
Bushwacker is running an etec 150 on his 20 Seafari and seems very pleased, so I'm sure he'll chime in at some point...