"On a nice plane at 2100rpm. Will the MPG be higher????"
Yes - the fuel used to cover speed over water will increase by twofold or more on your boat if you move up from 6-7 knots. Here are a few past sources for you to read about your early model 34 (I had a 78 myself - great boat).
BoatUS - Boat Reviews - Mainship 34 Motor Cruiser
http://www.trawlerforum.com/forums/s...dels-4065.html