Ok, I've read through the comments..
Some thoughts..
It makes sense that as boats have become deeper in the vee, that outboards would need to sit lower, so longer shafts to keep the power heads clear make sense.
Before I chopped the transom off on my new project, I researched the standard transom heights. Seemed to keep coming back to 6235 mm which is 25 inches. Now the transom I just took off, I had at 680, and the motor sat quite high off that. Had approx 12 degrees on the plate and if you extended that angle it was very close to the vee of the hull at the bottom.
This all put me in a mind spin as to what I should do, don't want to chop a transom too low only to need to raise it again.
Asked my son who fits motors, their jig sits on top of a transom, no reference to the bottom of the boat, this top edge is determined by the manufacturer, the further back off the hull, the higher it must get.
In the end, I wheeled the motor out(I made a stand with wheels), parked it up against the boat and yep, it seemed with the motor at 90 deg to the hull line, 635 was correct. I chopped it there.
Yet to fit and try it out.
So, Steps, if a boat has a pod, I'd assume it is already raised to allow for the elliptical curve of the water coming off the hull, and a 20 inch shaft will still be too short. Look at the height the current motor is at, yes you might be able to put your 20 inch on 2 inches lower without chopping, but that might not be enough. Some props seem to cope with ventilation better than others, think surface piercing..that might work..
All in all, I think you are better to stick with a 25 inch motor on a 25 inch designed boat. Keep an eye open for a dead donor motor for a 25 inch leg..?
My son recently picked up a 90hp yammy, when we looked, it had a 25 inch leg, spacer type..not the 20 advertised. But we thought hey, we can stick 5 inches on, he's a boat builder...turns out his boat had a 16 inch transom...damn...we have found a dead 20 inch 90hp, are negotiating acquiring it as the rest of the existing one is serviceable.
you can't fix an idiot with duct tape, but it does muffle them for a while...