A small point, pard Mad Dog:
The crescent buttplate is NOT supposed to go tucked into your shoulder like we learned ... it's supposed to go over the joint - where the upper arm connects to the chest. Try one - it fits PERFECTLY. The recoil from a pistol-caliber rifle - especially out of a heavy Henry or '66 Improved Henry is pretty negligible. That goes for traditional power loads as well as modern smokeyless loads. In other words, it ain't a problem!
As for an original caliber, you've got real good answers already, but ... 'tho it wasn't original to the Henry & '66 rifles at the time, the 45 Colt, 44 WCF [44-40], 44 Spl, 44 Russian, 45 S&W [45 Schofield] etc. are all period-correct calibers, so you should be able to find something that suits your taste ... and wallet! Many of us have picked our calibers so we carry the same caliber in both rifle as well as revolver. Modern technology has allowed us 45 Colt afficianados to have rifles chambered in this caliber, 'tho that NEVER happened "back in the Day!" (to my knowledge - at least on a mass-production basis.)
I shoot Pistoleer very often, and my C&Bs are an 1860 Army (Uberti) and a 2nd Gen. 1st Model Colt Dragoon. I shoot my '66 Winchester [Improved Henry] and my Tula 12 Ga. hammered double. LOTS of flames and beautiful white smoke from the large amounts of BP I feed my weapons with!
Have fun!
If you DO get the '73, you can shoot it in ANY division ... except Pistoleer. If I only want to shoot 2 guns, instead of what Pistoleer requires, I enter the Working Cowboy (or as I call it ... Working Pistoleer {LoL!}) class. You don't HAVE to shoot centerfire revolvers in WC.