I know that an 1/2"API thread form is exactly the same as a 1/2" NPT thread to ANSI B1.20.1. We supply this thread type in a number of subsea valves to API6A PSL3/4.
I don't know about the bigger thread types as never doen them but seen them on items such as tubing hangers etc with end users. I would suggest you talk to machine tool supplier such as SECO or good thread gauge supplier - recommend you contact TRU THREAD gauges in the UK, they are both UKAS and API accredited for gauging. John Glover at TRU THREAD should be able to help you.