Thanks for the replies, guys. Daryl, I had to laugh at myself--I had to look up the powders you mentioned. I was considering powders more in the range of IMR 3031 to IMR 4064. Those have worked well for me in a couple of 8x57s, so they seem a good starting point.

I got home from work tonight and plugged the lead pot in. The pot was full of soft lead, but figured that would be close enough to give me some measurements. Once the lead and mould were up to temp, I cast about a dozen bullets and called it good.

The gas check shank mic's out at .3408", with great consistency and concentricity. As I feared, that works out to a 35 caliber gas check. The mould is old enough that 9.3 checks were likely unavailable, leaving the options of using 35 cal or 375 cal checks.

Current plan is to finish lapping out the sizing die while I wait for some 9.3mm gas checks. I'll ream the gas check shank of the mould to fit the checks, and then I can get to load development. (Since I hadn't actually planned to get a mould til spring, I'll still be ahead of my mental timeline. I must be forgetting something. . . .)