Re: 1964, 16 foot, MFG w/ 25 hp Mercury
Welcome!
I have a 1960 Oxford, a fifteen footer, on which I have a 20 HP Honda. I use it mostly for cruising around a small PA lake. Usually its only me (180 lbs) or my wife and I both (300 lbs) and the boat has a decent turn of speed. Alone I max out about 23 mph. Cruise is about 18 on flat water. With both of us it's slightly less.
My engine is a long shaft type and should really be a short one. This does cause some excessive drag. It's just not worth it to me to change engines.
You boat is larger and heavier than mine. More experienced owners here have given you some good info. How much power you need depends on how you plan to use the boat. I set mine up to accommodate my state lakes restriction to 20 HP max. If I weren't constrained by that, and I used it in bigger water, I would have went with a 40 - 45 Honda. On yours I think maybe a 50 at the minimum. Don't forget, proper propeller selection is important to overall performance. On my Honda I have a 9 1/4" x 10" prop.
I'm happy with mine the way it is. It suits my needs and came at a very low cost.
See my thread for more information.