According to a post on another site from someone who contacted the Howa importers about the 223rem twist rate, serial numbers from B312235 onwards are 1-9" twist.
There will be some old stock so you may need to specify an order.
I've got the DNZ GameReaper mounts on my Howa 223rem Heavy Barrel. They work very well, you should be happy.
The rifle was set up with similar purposes to yourself but i found that the 24" heavy barrel was too cumbersome in the field so had 6" taken off the end. Made an amazing difference to weight and pointability of the rifle. Still not light but manageable to carry around and even shoot off hand.
It is also a 1:12 and shoots 40gn acceptably and 50-55gn quite well indeed. Haven't tried any heavier.
If you question accuracy of a shorter barrel i'll post a pic of the first test run for recent load development. You might get the barrel lopped when it's in getting bedded and the trigger done. Might cost $100. Just an option to consider for making the rifle manageable in the field,on foot.
Cheers