One of the things I'd like to comment on is the common perception that 8-40 screws are stronger than 6-48's. More than one engineer has told me this isn't true, because the larger threads on the 8-40's reduce the diameter of the screw at the bottom of the threads. The one real advantages of 8-40's is that slightly off-center 6-48 holes can be re-drilled to 8-40 to fix the problem.
Personally, I'd rather have an action with built-in bases than either, but have fired a lot of rounds from hard-kicking rifles with scope mounts "only" attached with 6-48 screws with nary a problem.
Some custom actions do have nice features. The Montana 1999, for instance, is a combination of the best features of the 98 Mauser and pre-'64 Model 70, and comes in a left-hand version. The Ultra Light Arms action is very precise yet very strong, and precisely made from the ground up. Melvin Forbes once told me that one of the reasons he started making them was all the time it spent to get a 700 action light and precise. He figured it would be easier to start from the ground up, and it turned out he was right.
That said, I have a pile of rifles built on various commercial actions from 700's to FN Mausers, and am just as satisfied with those as the rifles on special actions.
