That is a very heavy load for a 75 HP and the motor will need a shallow pitch prop to achieve its operational rpm.
Propping is trial and error and the Hydrus series prop is made for pontoon boats that also need high thrust at lower speeds and good backup response. Give on a try in the smallest pitch which is 9"
Your goal for that motor is at least 5000 rpm at full throttle - 5200 to 5500 is even better for your application.
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.