Well, just in case you are not feeling clutch dog engagement….
First, I’m wondering how you converted to a 15" unit, didn’t you have to change the exhaust housing???? So, if you measured .005" on the OMC shim gage kit, that means you added .015" of shims, correct? Adding .015" shims would surely result in less backlash, so that seems in the ball park.
There is a lot of confusion with these engines, because some of them come with the smaller 40-50hp style gearcase, while the commercial models, like yours, usually come with the larger 3cylinder style gearcase. I don’t think you could inadvertenly get the small gearcase driveshaft to fit into the larger gearcase, the upper bearing set up is completely different as well, but you never know. Although I wouldn’t think the pinion from the larger case would fit properly onto the smaller style driveshaft. You didn’t change any of the other parts, so the driveshaft seems to be the only question mark…The only other thing I can think of is if you accidentally left out one of the thrust washers for either of the gears, or perhaps the thrust bearings. Did you leave out the upper thrust washer/bearing??
Nonetheless, check it over very carefully before trying it out. I don’t think the unit will last very long if in fact there is that much slop in the gear lash.