Yep, sorry I was at work on my phone posting that previous response. I just looked at Proof's website and it looks like all of theirs are small shank. You can call and ask to make sure, but most prefits are small shank. It's usually the ultra-magnums that end up being large shank, if anything. I'm not sure the lineage, or which calibers were/are large shank from Savage. I just remember trying to get a Criterion 6.5 or 7 SAUM prefit for my Mausingfield and they would only do it in large shank because they were afraid the wall was too thin in small shank.
I ended up just buying a normal Proof Research CF blank and having it fitted the traditional way.