Wolf, this is just my opinion, but I don't feel you need a jackplate to fine tune the motor height...the holes provided will get you close enough without the additional weight of a jackplate...You sure won't regret the additional 5" of height on the transom...good move!...
Look thru the Seacraft forum for ideas on how to raise the transom...there are some of the best pictures and threads there on fiberglassing...
http://www.classicseacraft.com/