OSU Sig: I am not familiar with re-barreling any of your candidate Rifles.
I am familiar though with several Rifles in 221 Remington Fireball cartridge and they are all WAY above average in the accuracy department.
I know of two Remington 700's in caliber 223 Remington that were re-barreled to 221 Remington Fireball - these belong to friends and are Colony Varmint Rifles - both feed correctly/reliably - I will have to contact them to see if the magazines were or were not modified to the 221 Fireball.
In the past I bought 2 (two) new factory offered Rifles in caliber 221 Remington Fireball - again both shoot VERY well indeed.
One is a single shot Kimber if Oregon Ultra-Varmint and the other is a Remington 700 Classic in 221 Fireball - it feeds fine from the magazine.
An old friend of mine about 25 years ago contacted the Remington Custom Shop and begged them to take his money and make him a Remington 40XB-KS in 221 Remington Fireball. Finally they did and he let me shoot the Rifle at the range on a dead calm day. The groups I (we) fired that day are some of the best groups I have ever had anything to do with from a "factory Rifle"!
I guess the object of my post is to urge you to give this cool little cartridge a try! Its efficiency, accuracy, brass life, barrel life and low recoil are wonderful attributes in my experience.
Finally I like the 204 Ruger cartridge immensely as well, and seeing's you have a pair of 223's I vote you re-do one of them.
Best of luck with whichever route you go - just GO!
Hold into the wind
VarmintGuy
