Thanks for your suggestion
The Catalina idea is a good one but I'm not sure if the hull rise towards the stern is approximately the same for both boats. What I'm mainly interested in here is the clearance between the propeller shaft and the hull.
The propeller information from CDI and Yanmar was specific to the 2GM20 engine and just assumed that there would be room for the prop on whichever boat the engine was installed upon. Elsewhere I have read that for proper operation there should be about 20% of "headroom" between the arc of the prop and the hull. That would mean that CDI's 16 inch urethane prop would need 9.6 inches between the hull's bottom and the center of the propeller shaft. The 15x11 prop Yanmar recommends would need 9 inches. I've spoken to a couple of Hunter 34 owners at the marina, one with a 2QM20 and the other with a 2GM20. Both have 15 inch props on their boats and seem happy with them.
I was hopeful that someone who owned a Helms 30 could report on what worked well and what didn't from personal experience.