I would have the block threaded by your manufacturer.... You can check the threads by threading it into a factory 2260 tube.... Then boring out the shortened tube and tapping it with a 13/16-28 tap should give you perfect threads.... I would personally not be comfortable doing internal single point threading of such a fine pitch, as it is very difficult to hold the required tolerances, unless you have lots of experience.... A tap, as long as it is held straight with the tube.... and providing the tube is bored to the correct size for threading with your required thread.... is a LOT easier, and should always end up within tolerance.... JMO....Bob
May I ask what machinist you have contracted?I too would like to get a tube cut down. The only machinist that I'm certain has the knowledge of stress factors of the threads under pressure is Dyotat100 ( Doug) but I don't think that he is still a member here.I tried the order from Crosman custom shop, but they don't actually build custom stuff except assembly of guns from stock parts.Hunter