.750X20 TPI, If I remember correctly I think the last one I did, I turned the shank to .746.
On the length, get that from measuring from the receiver face back to the bolt face, and deduct what clearance you want from the bolt(not the counterbore for the case head) to breechface. A .038 counterbore in the boltface and a .005 clearance will give a headspace of ..043 and that seems to work well for most rimfire match ammo.
Back in the '60s, The American Rifleman magazine published several lists of diagrams, with OD, and tpi, specs. for numerous hand, and long guns.
Regards,
Bob