In simple terms, the ANSI B1.5 ACME screw standard applies to single start screws only. Once the lead angle of the thread exceeds 5° as with 2, 3, 4 and more starts, some interpretation of the normal flank angle and internal and external thread limits of size must be made by the screw and nut toolmakers and this varies widely by manufacturer. It is always risky to procure the nuts and screws or tooling therefore from different manufacturers.
Can you send us a sample of the screw and nut(s) you are purchasing?
Can you send us drawings of the screw and nut?
What is the overall length of the leadscrew(s)?
What is your yearly quantity of leadscrews and nuts?
It is unlikely that you will have success with hand tapping these nuts. The lead angle is too high and the parts will tend to ream rather than self-feed. We do this type work on large stroke, specially built CNC leadscrew tapping machines and you would need something similar that can deliver very high torque at very low RPM with the spindle synchronized to the tap slide. Do you have a machine similar to this on which to do the tapping operation?
Since these type products are what we specialize in, I am sure we can offer you something once we receive the above and have had the chance to do an engineering review. For example, we have a 5/8 X .500 size in our OEM HiLead line which may work for you. You would get 25% faster linear travel for the same RPM or you could decrease the RPM by 20% and produce the same linear speed. Generally, these are easy accommodations to make.
We look forward to hearing from you and working with you on this project.