Hi Connor,
If you're gonna carry another 1000 lbs, which sounds typical of Bahama trips, the props you have may be about right! I had my spare Al prop cupped so that it would turn about same rpm as my stainless prop; when I got over to the Abaco's I would switch my good stainless prop for the aluminum prop to prevent reef rash on my get-home prop!
The rpm for peak efficiency is primarily a function of the cam but the amount of gear reduction and prop design will also affect it. A 4 blade prop will sometimes be more efficient at cruise because it has less slippage, but higher friction losses at high rpm tend to hurt top speed. The duoprop may have similar issues, but it should eliminate all the swirl in the prop wash which should be a good thing.
Both the diameter and length of the tubing will affect TRANSIENT response of any pressure gage, but since there is no flow going thru the line at steady state conditions, it won't affect the reading. When I first hooked up the vacuum gage in my truck, I tee'd into the vacuum line running to the power brake booster with a 1/4" line about 2' long and that was too sensitive - the gage would fluctuate a couple inches as each cylinder fired! Had to put a .060" dia. orifice in the line to stop that!
Sounds like you might want to move those exhaust risers up more than a couple of inches! Having them too low is a quick way to kill an engine! I'd never add weight to a boat just to balance it, but have you considered adding a ballast tank? It would damp roll as well as help adjust longitudinal trim. (Check out Moesly's patent
Moesly's Patent on Carla's web site.) He used ballast tanks in the race boats; they worked well and he supposedly designed the Seafari 25 with the idea of using a ballast tank, but sold the company to Potter before he ever built a 25 and Potter evidently never included the ballast tank.
Denny