Just a little update to this thread, since so many people provided suggestions and help:
I'm an idiot (knew that already, luckily) and when testing if the transmission slipped, was turning the output coupling to the right for my right hand prop. If anyone finds this thread in the future, that's the opposite of the direction the force actually acts on that. The transmissions will slip in the direction opposite of what it's meant to turn, and that's part if the design.
So once I had the transmission out and on a bench, I removed the shifter side plate cover and checked the insides: a moderate amount of worn clutch plate material on the bottom but nothing excessive at all. Upon reinstalling the side plate cover, discovered that moving the position it's mounted in (It has oversized bolt holes that go over studs on the side of the transmission) was able to adjust the shifting point, so now it properly "clicks" into gear
in both forward and reverse.
Also, over the course of a 24hr haulout, discovered that the cutless bearing was entirely ruined: It had apparently overheated at some point in the past and the rubber slotted sleeve had melted to the shaft and was providing a huge amount of resistance to turning the prop as it rotated inside the brass outer sleeve.
I have yet to reinstall the transmission, but I'm hoping it'll be working properly now that these multiple issues have been solved
. Will be changing the ATF at frequent intervals for the first little while, I think, but hoping it'll work
for a while. Currently waiting on an engine
mount that was also discovered during the whole process (which may have also been part of the issue: alignment problems exacerbating the cutless bearing wear, which then failed and caused transmission problems.
Will update once I have it in and working again if anyone's interested!