Please dont use any fillers and chance a chamber ring. Yes, some have shot 'thousands' of rounds without trouble, but there is far to much data regarding this to ignore nowdays. The rifle you wreck will be your own.
I doubt the case side split was due to using a fast powder, if so nobody would be using it any longer after ALL these many years.
Drawn brass, in the process of making cases, goes thru stresses that create weak spots, thicker & thinner, throughout. It is amazing that it holds up at all. So, given that, and say, an overly large chamber, then subsequent resizing (down) and blowing back out in repeated firings, something has got to give.
I would shoot those Unique loads without hesitation to get aquainted with the rifle, if nothing else. Chamber a round, tip the muzzle up to orient the powder to the rear, and carefully lower to sight. Enjoy!