I second the recommendation for Match Grade Machine, excellent barrels in my experience. As for twist if you went no heavier than 62gr you may be able to get by with a 1:10 or even 1:12. 1:9 should be close to a sure thing up to 69gr.
As for grips and forends, the above recommendations are all good and I would add Tonys Forends and Grips as well.
http://www.tonysforendsandgrips.net/I ordered a "semi-finished" forend from him for an Encore rifle that turned a 2MOA stringing 17MachIV into a reliable sub MOA shooter. I ordered the semi-finished version to try and match the color and finish of an existing buttstock. It came pillar-bedded and fit almost perfectly. The only finishing required was a quick wipe with 600grit paper and staining and finishing to taste. I was and am pleased.
Check out specialtypistols.com as well as there are many informed opinions on there and probably someone who has already done what you are trying.
You have probably already considered this but a 8 inch 223 is going to be LOUD!