Ron- I basically stole candutch's idea for the rear casting deck and I'll give you my reason as to why I didn't make it too short. What I did was tilt the motor fully in the up position, then measured 3" from its closest point to the rear casting deck, to be sure I'd clear the motor while tilted. I can measure mine but I know I took off about 12" or so, and I used the original aluminum. If you never plan on tilting your motor, I don't see a problem unless you back troll.
Let's see what the pros say!