marksman, I see you're in MS. Do you know Don Geraci? He's in Independence, LA. Man, he's put together a couple of rifles for me based on 700s. Just threw a Krieger barrel onto the action, a little magic on the bolt face and viola?? Tack driver. Now, I'm talking a bit of time ago, but I spoke with a friend not too long ago and I'm hearing that Don is still building rifles. I think he just built one for D. Trump Jr. or some such thing.
Anyway, he can do full blown custom or doctor up what you have. I've shot lots of rifles he's done. 70's, 700's, Savage actions, the all shoot really well. Just make sure that what ever action you choose, has the triggers, stocks, mag systems, doodads that you want available for it.
As for custom actions, in my limited personal experience...I've always been a fan of 700's. I can take them out and treat them like a tool and expect them to work. With the custom actions, they are kind of like a girlfriend. You have to love them and treat them right or they'll give you trouble. You can loosen the specs up a bit to make them not so troublesome, but by the time you do that, you could have just used a stock action. But...believe me, I'm no rifle builder. talk to someone that knows.
But man, the last one that he put together for me, I wanna say it was a 1:10 twist krieger barrel. Anyway, he cut the chamber so tight that when I went the resize the brass, there was almost to resizing to be done. Cheap steel case ammo would even chamber...no, I wasn't going to shoot it, just wanted to see if it would chamber. So, my buddy chuck hands me a handful of rounds and says try these. Turned out to be 168gr Berger hunting VLDs, Win brass, CCI benchrest primers, reloader 15 powder. It was only 100 yards, but takes note of where the first shot landed. Then he calls misses on my second and third shots. I'm seriously scratching my head. So, I take my fourth shot and this time, chuck has his eye on the spotting scope while I'm shooting and says...wait, I saw the paper move. Same on the last shot. Turned out, all five rounds went through the same hole. I didn't even know how to measure the group, except to say that it was one hole slightly larger than .30 caliber.
Anyway, if you are looking for an ultra accurate rifle, see what your smith says about the downside of going with a custom action. Or call Don. Google him.
Really, the only time I'd consider a custom action is if you're competing for groups and measuring them with with a micrometer.