DON - What I did to my 50C last spring was take it ALL apart, the main backbone of the 44 & 50's is a stamped 1/4" thk piece of steel bolted on top of the deck skin. To get it apart like that you remove all the bearings & housings, the idler & idler pivot arm. I check EVERYTHING for flatness, and true the skin to the 1/4" plate.
There really isn't anything you can do to the spindle bearings, the tapered roller bearings for the old decks won't work. Like Wes mentioned, just make sure the bearing is tight in the housing when assembled.
With everything back together install the blades and make sure each blade tip is even with the blade tips next to it, set a straight edge across the tips of the blades and make sure they are all at the same level. If one blade is lower than the other two you may have to shim the lower blade with a thin flatwasher. If a blade is lower/higher in front than in back the spindle housing or deck skin or the 1/4" plate isn't flate enough yet. Also check both ends of the blades when checking everything, you may have a slightly bent blade that's throwing everything off.
When you have the spindles apart, I would clean all the old grease out, then grease it up good when it's back together. It's probably O-K.
Your 1512 won't even break a sweat with a 44" deck. I ran a 44A on a 129 for 15 years. Tractor was a "Basket Case" when I got it, a real money pit, but was my favorite tractor to mow with. Wish I'd kept the deck when I sold the tractor 4 yrs ago, I'd put it under my 72.
Biggest problem I have with my 50C is the fact it's so wide, and it's touchy to get it level side-to-side since the eyebolts & rear mountes are only eleven inches apart and there's twenty inches of mower beyond those mounts! I added a roller under the back of the center blade inside the mower housing and it really helped improve the way the mower performed.