Check with Paul at Barnwood Arms in Ripon. He might have a set of gauges for the 7.62x54R. Also check with Sierra Arms, but I perfer Paul. He has a lot of experiance.
Headspace on a rimmed cartridge isn't a big issue, not like a rimless round. The headspace on your rifle is just the gap in the chamber where the rim sits. What you should ask them is "is it safe to shoot?" They can check the locking lugs, the bolt, the receiver, the chamber, the bore, etc.
I don't know what they charge. Call before you drive up there to make sure he is in.
You only live twice. Once when you are born and once when you look death in the face.