Fountain used 1.65 drives on the 42 fountain when the had mild power. I assume the larger pitch prop provided better performance. When going from 4 to 5 blade props there is around a 2-3 pitch lower to keep same rpm. Would going to 1.65 drives when going to 5 blade be better for top end performance because you could keep same pitch in props?

No, keeping the same pitch and changing the ratio is rob peter to pay paul. The only difference being the characteristics of 4 vs 5 blades (slip and lift). Mercury only recommends changing gears when there are no more available prop pitches. with the price of gears and installation I do not see any advantage.

My gun has ssm 3 drives and my primary gear ratio I use is 1:40 but last year at the beginning of summer I had a problem with a drive so I bought a complete spare set of drives but they are 1:68. Tried 4 sets of props and no matter what seemed to be 3-5mph slower. Could be partially because with the 1:40 I get on plane easily but with1:68 there is so much torque at the prop I had to run larger diameter or it would just blow the props out of the water. So even though I am able to spin a much higher pitch prop I feel I lost performance if anything. Just got my hurt 1:40 drive back so needless to say as soon as the boat comes out of storage they will be going back on the boat.

I would keep the 1.5's and drop the pitch if you go with 5 blades.

I don't think you'd see any real differences in cruise or top speeds with gear changes and taller props.
I have the opposite. I spin 24's with 1.36's. No real differences with my speeds vs other 32 AT's.

You need to try the propellers it may not be 2-3 pitch numbers off. Going from a 4 blade to a 5 blade CNC prop might surprise you. CNC propellers are faster.

Wilks has a 38 Donzi with 575sci's and Bravos