When going with a custom barrel, everything revolves around the exact cartridge you want to shoot. You have already determined you want to stay with the 270 WSM (a great choice). According to Berger, this bullet will require a twist of 8.3", but most barrel makers will either do an 8.0" or 8.5". I would go with an 8.0".
Next, you need to take a look at the reamer that will be used. Most gunsmiths will have the standard SAAMI spec reamer in 270 WSM. It is possible that your smith may have a slightly different reamer that takes a little slop out of the SAMMI spec chamber (probably not). Although a tighter chambered reamer is not necessary - research on reamers is a good way to learn more: start with Pacific Tool & Gauge.
You can tell your smith that you want to shoot the new Berger 170 gr. VLD and he will throat the chamber accordingly, but the best way is to mock-up a bullet the exact length you need and send it to him. This will require you to either live with your current mag length , or to possibly try to extend it. I would start with Wyatts Outdoor. I know they have an extended box for the Win long action, but I am unsure about a short action. Give them a call to see what is available. Sticking with a length that will fit your current box will rob you of some case capacity. If you cannot extend the box (either not available, or too expensive because you may also have to replace/machine your bolt stop or extractor), then I would simply choose the max length that your action will extract and live with your new rifle as a single shot.
