I have a '74 Johnson 115 on a 16' '74 Starcraft American bowrider, It is a fibreglass runabout v hull boat. Not too heavy. Is yours Aluminum or Fibreglass?
My boat came stock with a 19 pitch aluminum but it was too big for the motor on that boat. We put a 17" Al on it and that gave us the desired 5500 rpm at WOT. We replaced that with a 15 pitch SST and that also gave us 5500 rpm at WOT. The boat is great for waterskiing and tubing. Great hole shot. Goes about 35 at full speed which is faster than what I water ski at which is 32 mph.
You want your engine to hit a max of 5500 rpm at full speed. If your current Al prop gives you 5500 at WOT, then you should probably drop the pitch by 2" to go to a stainless steel. Like I said, a 15" SST works great on my boat with the same engine. Don't know how similar your boat is to mine...