TOPIC: Short shaft for '92 Merc 3cyl 75 HP

My engine (Model # 0D230618) is a long shaft. I'm running it on a Glasspar G3 with a 15 inch transom (1960 vintage.) I have it mounted in the second from the highest transom bracket mounting holes which puts the top cavitation plate about 1/4 inch below the hull bottom. Mercs of this vintage have a square design on the leading edge of the shaft housing the bottom edge of which sits just above the top cavitation plate. My problem is that when running under power from about 20 mph up, water runs up between the hull and the lower unit and spills into the splash well. I've jury-rigged a heavy rubber shield that stops all but a little bit of the splash, but the fact that it is there makes me believe the engine is still mounted a tad bit low. I'm wondering if converting it to a short shaft (if they are available) is difficult to do and what some likely sources for a short shaft might be. I'll appreciate any suggestions/ideas/wisdom. Thanks.