1971 Evinrude - Wrong prop?

I have a 1971 Evinrude 25hp mounted on a 14' semi-V with wood decks and floors, it also has a trolling motor and battery. I would guess the total weight of the boat loaded to be @550lbs. With 2 people in the boat I'm getting maybe 18-20mph speeds. It seems like it accelerates slowly and even at top speed it sounds like the motor isn't running enough RPM's. Both cylinders are getting spark and based on the appearance of the plugs after running a new set and pulling them after that both cylinders are firing.
My thought is that I may need to step down the prop pitch for better performance, is there any reliable method of determining the approximate engine speed without investing in a tach that I have nowhere to mount permanently on the boat?
Are there any pitfalls to trying out a lower pitch prop other than being careful not to over-rev the motor?

tach is the for sure way. You will have to use your own judgment on the over rev part.

It does not hurt to try a lower pitch prop. You need to know what is on there now. My guess you might end up around a 9" pitch. Lower the pitch, you may not go as fast, but will be quicker to get on plane and max speed.

That sounds slow. I get approximately that speed with 2 guys in my 15 foot jon with my 18hp.

Had it out yesterday and surprisingly it ran a lot better. All I've done so far is to check the adjustment of the carb and throttle to make sure that twisting the grip actually opens the butterfly all the way and get it to idle nice. Then before the last trip I added some Seafoam to the gas/oil premix, It has had about 5 gal of the Seafoam treated gas run through it now and actually seems to run noticeably better. I guess that sitting in a closet for 20+ years can gum-up a motor.