Cool project.
More specific info about total weight, length, number of passengers would be helpful. My data shows 1956 Voyager Hardtop is 18′ and 510 #dry and rated for 60 hp. Transom width 54". Any extra floor weight? Fishing boxes and gear? etc.
My Alumacraft Super CS is 15′, 400# dry, Transom width 58" with twin 57 35hp Javelins. So your boat is 3′ longer, about 100# heavier with narrower transom (and slightly wider amidships). I use twin AJC 465 (10 diam X 16 1/2 pitch 2 blade brass). I get 35 MPH with two people and about the same with 4-5 people. About 30 # of anchor, tools, and life jackets. Humongous torque and instant hole shot with twin blades and 70hp. Mid speed acceleration from fuel saving cruise throttle to WOT is a neck whipping thrill. Great fun!
Assuming both your 35’s are fairly low hour (about 120-130psi) and properly tuned I am guessing you could safely run twin AJC 460’s (10X15) or AJC 461’s (10X 15 1/2). AJC 465’s may be a little too much for your larger boat and heavier load.
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