.740 is the is the gas block size needed.
.750 is the Gas Block size, as per JP website.
There are multiple barrel sizes listed here. You would want to match the gas block journey size of the barrel to that of the gas block. Additionally, some people also want to match up dimples for the gas block to gas block set screws, or have other custom work done by a gunsmith to ensure the gas block is very secure on the barrel.