Well now that I know your boat and engine year model I have a couple of comments:
Have no idea what Mariner was running in '78. If 3 cylinder may be loop charged and prop rated or more likely 2 cyl and cross flow power head rated. Difference is 10-15% more prop hp on the prop rated engine.
Googling Sea Ark shows it to be an easily planing hull alum boat. That and your posted 27 mpg make sense. Your rpms I now find out were just a WAGuess. That's good because something in your other post I answered this morning was wrong as I said. The position of your AV plate vs the bottom is just right in my opinion and I am running a similar (sort of) boat.
What you hear and facts need to be verified as something like the tiny tach can tell you. I googled them the other day and they are simple and easily mounted. My lawnmowers use them and they have to be cheap to purchase to put on a riding lawn mower.
Not knowing more than I do, I think you are getting all you are going to get out of your rig. Unless you just want to fool around and throw money at it I doubt you will do much.
Looking for a 14' boat on the their www the 1448 which is a 14' john with a V at the bow, 14' long and a 48" bottom width, 67" beam is only rated for 30 hp. You didn't specify which Sea Arc you have but you need to watch max rated numbers. Overpowering if you choose to increase hp may not be the right thing to do as it can cause you lots of problems.
HTH,
Mark