Hey all, I'm happy to give an update.
The short version: so far, everything is perfect!
The long version: I've put about 30 hours on the 336 since the overhaul. I've graded my very long driveway a few times, rotary cut my neighbor's field, rotary cut quite a few saplings down in order to begin making a pasture for my horse, tilled my garden (post-growing season), disced my front yard in order to reseed it, smoothed it out using a back blade and moved some round bales. All of this in addition to using it for regular lawn mowing using a finish mower. I'm excluding all of the tiny things I use it for, like using a boom pole to move heavy objects, like when I use my table saw in other areas of my property. Those little things are what really makes my tractor great to have around.
All in all, I'm very happy with the results of my rebuild, both the aesthetic and the mechanical side. The engine runs perfectly, idles well, only has visible smoke when throttling up and, even limited to 2nd and 3rd ranges, it has enough power to use a 6' tiller and rotary cut at rated speed. Was it all worth the time and money? Just the experience of it all was worth that. A college course on diesel engines would have cost more in time and money. As a benefit, I get a working tractor once the final exam (initial start-up) is complete.
Moving forward, I still have some things I'd like to add, like rear remotes and fixing the 1st range, but funds dictate that stuff and I'm in no rush.
Thanks for the curiosity. And thanks again to everyone who helped ensure it turned out this way. Restore threads are bitter sweet in this way. They are vast adventures which, if done well, end abruptly and don't come up until the next issue, which hasn't yet occurred.