18' SF Performance Questions

[otterhound]

I finally launched a 1978 18' SF I picked up last fall. It has a 1992 Evinrude 120 hp which after rebuilding the carbs seems to run well. The compression checked out at 128 psi per hole so it seems the bones are good. The prop is an aluminum 3 blade in good shape and it's marked 13 1/4 x 17. The max rpm I've seen is 5000 at 37 mph and 4000 will give me right arround 26 - 27 mph. It also seems to fall off plane around 18 - 19 mph. The 5000 rpm max seems low to me although I'm happy with the cruise numbers. Is it possible that the motor is over propped? I'm willing to grab another prop but unsure of which way to go. Also the boat does not have trim tabs. Thanks!
Rod