I have a 17' procraft fish'n'ski with the same 1978 115 Johnson.
I have 3 props a 17" 19" and a 21"
The 17 I also get 45mph gps.
For the smaller lakes in Indiana I use the 17 gets me up on plane quicker.
When we go down to Green River I put on the 21 to cruise with. It take a little longr to get on plane but it will run 55 mph gps.
EBAY is a good place to pick up props pretty cheap to experiment with if you take your time should be able to pick up some for less than $50.