Originally Posted by Buccaneer

Generally speaking, the sweet spot is going to be in the 3.5" propshaft below pad ballpark. Although I have 6" jackplate, on my Stratos 285 (18.5') with 150 it is at 3-3/8" below pad. Hopefully your bolt pattern will let you get near 3.5". Long term however you are better off with a 6" plate if you can find one used somewhere.
Lots of setup info here - http://www.bbcboards.net/zeroforum?id=11