I had sent an email to Stu at Stu's Go-Devil about the 20 inch transom, this is what he had to say.
"The higher transom raises the angle of the cavitation plate, over the prop, enough that it will try to push the prop out of the water. To run the motor in this configuration you will have to hold up on the tiller handle. The faster you go the harder it is to keep the prop from popping out of the water. In addition, the higher angle of the drive causes a small decrease in thrust. A trim tab can be bolted or welded to the rear of the cavitation plate and bent up enough to keep the prop in the water, but I found this solution made the prop pop out quite easily and still decreased thrust. Any hook or roughness in the bottom of the boat, near the transom made it even more difficult to hold the prop in the water. My preferred solution is to cut down the transom to 16 inches. I find the center of the transom, from side to side, measure 9 inches out on each side of center then mark a 45 degree angle up to the top of the transom. Using a sawsall I cut out this piece of the transom and cover the exposed surface with new aluminum. On some boats the knee brace from the transom to the floor will also require modification. The best guy in town, that I have found, to do this modification is Karolds welding off Davis Road. If you have some experience working sheet metal and/or welding aluminum you can do it yourself, because it certainly isn't rocket science. Please don't try to run it on a 20 inch transom without, at least, adding the trim tab. It will wear you out trying to keep the prop in the water. That boat/motor combination is an excellent choice and while it won't be fast you will be amazed with the fuel economy and reliability."
The weekend is here so I'll have to wait until Monday to see how much the modification to the transom would cost since I'm not set up to weld aluminum. I do have a little Hobart now, so I'll try putting a trim tab on the cavitation plate and see how it performs like that this weekend.