The Heaviest isn't always the best. You can get to the point where you can't move them fast enough to open at any distance.
As far as the 1-9 twist. The ability to stabilize bullets has to do with their length not weight. When all you could buy were cup and core soft points weight was useful, but with mono-metal, extreme aerodynamic shapes and plastic tips length comes more into play.
I shoot a 1-9 22-204 and it just dotes on 40gr NBT's and that bullet at around 4000 fps has been my goto pelt hunting bullet for a long time. It is so accurate and performs so well that I haven't even bothered to experiment with anything else other than some Lead-Frees.
Instead of looking for the heaviest bullet for the 223 I'd be looking at a bullet that was designed for hunting larger game, Nosler Partition, Win. 64gr Power Point or one of the Barnes Monos, I'm sure there are a couple others. Bullet design is quite important, I'm old enough to remember when the 243 hit the shelves and the hoopla about it being a just the Hammer of Thor on deer, but it's bullets were a problem, we were getting in to higher velocities than in the past, I had problems with heavies not opening at distance and gave up on the 243 as a big game round and went back to the 250-3000 and 257 Roberts both with decades of big game hunting under their belts and moving at a slightly slower pace where cup and core bullets shine.
As far as building a 25-222RM/25-204, great cartridges, I've been shooting the 25-204 successfully for 15 years BUT it needs the same length action as a 250 Sav. or 257 Roberts. I built mine because I had a .378 boltface action laying around and needed a deer rifle for that fall and it was cheaper to build than to buy a new rifle and there were no really petite actions in a deer capable cartridge at the time. Today I'd have built a 25-22 Nosler, 25 Grendel or just go with the 6.5 Grendel and build a petite little deer rifle on a Mini-Mauser, CZ-527 or Howa Mini. I have a Howa Mini in 6.5 Grendel.
Good luck with your search.
First deer I took with the 25-204
He was a big one and a good test for a new cartridge the 100gr Partition worked perfect.
Howa Mini in 6.5 Grendel a handy little rifle