The Cimarron has at least a ladder adjustable rear sight which will get me entry level lead lobbing. Dies secured, and THAT was a search. 100 cast 300gr ordered and on the way. Brass and LR primers will be a challenge. Powder. Searching my manuals, and online, I've settled on unobtainium H4198 and 5744. Why? Best combination of accurate and low recoil/pressure for 300gr and 405 gr loads that I can discern. As always, the hunt is almost the best part. Once the dust settles I'll report results.
It sounds like the road is laid out for you... good job.
Question: You said RCBS dies... are they Cowboy dies, or standard dies? If they are standard dies, you should look into something like the Lyman M-die to flare the case neck for the bullet. It works a little different than a standard flare die, which is fine for jacketed bullets, not so much for cast.
Brass won't be that hard to find... it's out there, even in retail locations like Cabelas, etc. I don't know how 'southern' you are in CO, but I don't think you will have a problem there, even if it's a trip up to the Springs or even Denver.
Bullets shouldn't be too big of a problem, either, there are enough online sources for cast. Personally, I like the 405's, I haven't had very good luck with LaserCast 350's for some reason compared to the 405's.
You picked the correct powders... although I'm an IMR guy, so I use IMR4198, not H, but that is a matter of preference, as is AA5744. I tried AA5744, it works, but not any better than IMR4198, for me anyway, and I use IMR4198 in my other cast rifle cartridges with excellent results.
Before you make a decision on the rear sights... shoot your rifle a bit, how you shoot it, and what distances you shoot may very well determine if you need a sight, and 'how much' sight you need.