7x57 Ackley Improvedlightbulb
That was my first thought, as well. If you are going custom, it makes sense to do that IMO.
I am curious to know, what is it you want from a custom rifle that you are not getting from your Model 70? I ask this in an attempt to gain understanding of what you are after, not trying to talk you out of the idea. Also, do your tastes run more toward stainless/synthetic or wood/blue style?
Here are my personal thoughts on the 7x57:
I think of the 7x57 as a quintessential light rifle cartridge. It is a versatile cartridge with a wide bullet selection and enough punch to take game larger than deer. Because I think of it as a light rifle cartridge, I believe it to be at its best in a light rifle (as opposed to a heavy barreled, varmint style setup).
I own two 7x57's. One is a scout style truck gun that I built from junk mauser parts. The other is a Ruger #1A. The truck gun has a 19.5" military barrel. It is accurate and is a fun little rifle. The Ruger #1A is about as close as I can think of to the ideal rifle for the chambering. It is light, nimble, and quick handling. It feels and handles like a quality shotgun. I don't own another rifle with that same feel. It is accurate, too.
All in all, if I were to have a custom rifle built in 7x57, I would go the AI route. I would want a sporter barrel no longer than 24". I would go with a nice walnut or a laminate stock with a slim pistol grip and forend (I have always found the schnabel type forearm on the Winchester Featherweights attractive). A walnut stock would get a hand rubbed oil finish. Wrap around fleur-de-lis type checkering would be on the list. Though I love the appearance of well done bluing, I think a matte finish is more practical. I would choose one that seals the metal as well. I would want aperature sights with a bead front. If the base gun were a bolt action, I would want the Winchester type 3-position safety. I would also want a nice trigger that breaks in the 2.5-3.5 pound range.
The real trouble for me with all of this, though, is that I like the factory rifles produced in 7x57 so much that I would personally have trouble bringing myself to commision a custom rifle so chambered. I love my Ruger #1. I envy you your Winchester Model 70. I also love the CZ550. All three are great rifles. With minor tweaks, I would be hard pressed to pass any one of them by for a custom. I could see maybe having one of them re-stocked, having the metal re-finished, or going with a custom barrel given the AI treatment at some point.
I dunno. You could go so many different routes with a custom rifle. What would
your ideal rifle in 7x57 be? That is the key question.