It's not a big deal, the reg doesn't need to be fully seated into the vertical ASA to seal and function. Just unscrew the reg one half turn, install hoses and gauge, and you're good to go. When the gun is gassed up the pressure will tighten up the reg and it won't feel loose at all.
If you want to be anal about it, drop an o-ring into the ASA before screwing in the reg. It should bottom out against the o-ring.
Nunquam emere quid potes facere