Major throws in a stretched out leather belt with extra holes punched in it.

Sell them both and buy a 44-40.

I don't have a '73 so no advice there. I've been shooting a Rossi '92 since 1992 in SASS matches. If you use real gunpowder (BP) you will either need the Snakebite "greasewagon" bullet lubed with a BP lube or some other generous lube grooved bullet like the Lyman 358311 (that I use) and two discs of medium brood core bees wax under the bullet. That load will shoot all day but shoots a 5-6" group @50yds. Accurate enough for SASS distances. If you don't do the beeswax discs then you need to do a wet swab "pull through" to prevent the last 4-6" of barrel from fouling after about each 10 shots (20" barrel). Plan on the pull cord splatting against your sleeve permanently instilling a black smudge.

I have used various subs (with the exception of Pyrodex due to its corrosiveness): Black Canyon, Black Mag 3, APP, Cleanshot, Clearshot, 777, and Black MZ. All of those listed work just fine with standard smokeless lubes. All of them grouped into 1.5" @50 yds. All except 777 produce about the same velocity as real BP. The 777 (I use 3f size) shoot the bullets about 1/3 faster than all the rest. The manufacturer recommends to only use FFg size 777 for cartridges but I had already worked up the load befroe reading the warning and figured the 357 mag is a pretty high intensity cartridge so I haven't worried about pressure problems. YMMV. Now I only use the other subs in 38 SPL pistols when I want light recoil. I will use any 150-158gr lead bullet and 15 grains (weighed) in the 357 and crimped in.

The Rossis are more particular about cartridge overall length whitch will vary with the shape of the bullet so I always work up a load by first finding the best OAL for any particular bullet. If the bullet causes hiccups then I won't use it. You need to start with extra long dummy rounds (~5), run them through the gun and seat the bullets a couple thou deeper and run them through again until they look too short and start hiccupping again. Write down the OALs as you do this. You will find the sweet spot and that is the OAL for that bullet.

357 Mag will have significantly less blow by than 45 colt.  Load your 357s with enough 3F APP to just below the base of the bullet.

Clean your rifle and lube with a BP compatible lube. Your 92 will easily run 18 stages over 3 days with just little Ballistol sprayed in the action at the end of the day followed by a wipe down of the outside.

Got back home from Alabama State, safe trip and amazingly I took Third Place in Gunfighter. Great weather there and travel home until the last hour when it rained/poured. Anyway, it is good to be home.