7.7 twist is not nearly enough twist to run the 90s, you will be limited to the 80-something grain class of bullets. In addition, you may want to consider your 28" barrel length further. As Wade's results prove, it will certainly work, but if you want to hit the highest accuracy nodes available to you, a 30" barrel will allow you to do so with slightly less pressure. Brass life when using those nodes is not so great to begin with, and the shorter barrel will either force you to run higher pressure than you might want to, or accept a slightly lower velocity. Anyhow, something to consider now as you're building it from the ground up and still have the option of selecting whatever you want. If the 28" will do everything you want, stick with it, but I would at least think about a longer barrel if you're going to shoot at 800/900/1000 very often. With a .223 at those distances, every little bit helps.
As far as the 5R, I only have one rifle that doesn't have a 5R barrel; it really doesn't shoot that well and I certainly don't compete with it. I use Bartlein 5R barrels in everything I compete with. The main reason for that is not because I have any proof whatsoever they're superior to Brand X Rifling Type Y, but simply because they have always worked very well for me...no reason to change something that is working. Both of my .223s have been shooting lights out with the 90s, the new one that I'm currently getting up to speed seems like it's really going to shoot well.
I am one of the minority as far as seating the 90s; I typically find optimal seating depth is .015" to .020" off the lands. More people have had success seating them into the lands by .005" or .010", even though they used the exact same reamer I used. Does that have anything to do with the rifling? I really have no idea why the seating depth should be so different when these folks are using the exact same powders/primers as I do, the same barrel length, and have chambers cut with the same reamer. What I do know is that the 5R barrels I am using shoot the 90s just fine. I also have another .223 in which I shoot the 80.5 Fullbore bullet, which wears a 7-twist 5R Bartlein and shoots extremely well.
Having said all of that, I don't think you'd go wrong with a 4 groove Kreiger either. Any barrel made by a top maker such as Bartlein, Kreiger, Brux, etc., ought to do the job quite well as long as you get at least a 7-twist. A 7-twist will be sufficient to stabilize anything you want to shoot up to and including the 90s. You may give up 2-3 % of the maximum [theoretical] BC possible by shooting the 90s out of a 7-twist, but it works just fine out to 1000 yd. IMO, there is no need to go as fast as a 6.5-twist. People have occasionally had issues with jackets coming apart using the 90s. Some have found that to be a function of not keeping their chamber/throat/barrel scrupulously clean. I always keep mine clean, and have never experienced this issue. However, I can only imagine that it might be exacerbated by using a faster (6.5-6.0) twist barrel. As I said, there is really no reason to go that fast as a 7.0 twist works just fine. If you really want to get every single speck of BC available from the 90, a 6.7-6.8 twist will be sufficient, but not any bit more than you need. Your chances of finding a 7-twist blank already available (in stock) somewhere are also much better than the chances of finding a 6.8 or 6.7 twist, which are essentially zero.