Check these out, too:https://www.gatewaytoairguns.org/GTA/index.php?topic=186333.msg156138947#msg156138947https://www.gatewaytoairguns.org/GTA/index.php?topic=129415.140The 1377 PCP looks great
I think that will work. Remove the reg output gauge and install plug just to get it out of the way. Something to cover threads on the bonnet. 5k burst disk on tank side.
Assuming you have to select a male-male nipple length, based from your mock-up, and what lengths are available, you will still need to do an install and leak check before knowing what your final gap for the barrel band is. NPT fitment is not exact when it comes to height. Allot of the Chinese fittings are all over the place with 1/8" NPT threads. Even tape thickness can change things.
Then there's this:https://www.gatewaytoairguns.org/GTA/index.php?topic=189768.msg156190329#msg156190329
100% understand if you are not comfortable with it. Just as an FYI though, aluminum CAN be tapped for 1/8” npt. Safely. Very common to do so. Dave