We had an older motor without that provision (might have been a 44ci Merc 400). We made an adapter plate from plate aluminum. Worked great for about a year but then the driveshaft splines started to fail. That adapter caused the driveshaft splines to not engage fully so Brooke Russell sleeved and pinned a slightly longer top end on the shaft. Over the years I suffered just about every common failure from breaking off the top of a crank to breaking out the rear bearing carrier in the lower unit and most of the stuff in between. I learned a lot and with the help of a few knowledgeable friends I got so I could make the 500s run pretty good and pretty reliable too.