Well, if the boat came with a standard (short) shaft engine, it must have worked out for the previous owner. In many cases a short-shaft engine will work, depending on how much powering you do, what sea conditions are, and what type of mounting bracket is on the boat. I assume that the 1975 O'DAY 22 had a lifting bracket? If so, you "may" be able to get the motor low enough to keep the prop submerged most of the time, however...the long-shaft engine would allow you to keep the bracket adjusted higher to keep the powerhead higher above the waterline. Depending on the make and model of the engine (and year), shaft extension kits are available from most manufacturers, cost will be in the neighborhood of $60-150 +/- and then there is cost of installation if you don't want to tackle it yourself. It may be more economical to shop around for a used long-shaft engine instead. Try a local Dealer or better yet, a Marine Consignment Store, you might even luck out and find a "parts engine" on eBay that would have the extension parts. I'd be carefull going that route....but it is an option.
On the other hand, if you will be powering in mostly calm water, and such.. you may be OK with the short-shaft.