Long Hunter Shooting Supply sells new slicked up Uberti Navies and Armies, which includes Slix nipples, the arbor length fix, lightened springs, and much more. Their anti-cap jam fix is an additional cost (pictured below). The '51s also have a 5" barrel option. I have a pair - they work flawlessly - and Long Hunter's is a terrific outfit.
A minor note for anyone interested in .36 Navies (51 or 61) Pietta vs Uberti is that the latter prefers the slightly larger .380 round lead balls which you either have to cast yourself or buy online (I highly recommend Whyte Leatherworks) as oppos