With that Montana 99 action you can go up to 3 inches for cartiadge length. I concure you need something between the 224 and 308 dia Several have been mentioned, and all are good. I personally think you give up to much going to WSM case, feeding issues, less mag capaticy and more money.

If it were me I would go for a round with some real class, great lines and lots of great history with it. I would choose the 7x57mm Mauser round. This is the ballistic equal of the 7mm08, or is it the other way around What better for a Mauser based action then a Mauser round, and it gives you the ballistics and bullets you choose orginally.

Just my thoughts and preference but you asked.

Too, eliminate all rounds with poor expansion ratio!!! A 358win is light-weight because of the big hole in the barrel ! I also like 338-284 & 338-08 ! Edward L.

MHO hit the nail on the head. The action is VERY long for a "short" action. Jim Brockman has been prototyping on the M1999 action since well before the first casting, and he said either 6.5x55 or 7x57 would fit fine. These would give you a bit of nostalgia (you certainly don't NEED another gun), and since you wisely chose Chrommoly steel, a beautiful deep blueing is possible. Something I didn't consider when ordering mine in Stainless. Pair that blue with a good stock blank, and you've got a reason to pull this "ole Bessy" out of the gun cabinet.
By the way, both of those rounds can be "stepped up" considerably with reloads, and there's always the Ackley route if you feel that need for speed again.

I'm waiting on my 6.5x55 LH action to be barreled and sent to Jim for the stock and metal work.

Safe Hunting,
Clint