constructive possession... sigh...

so i found a good deal on a 10.5in AR upper i intend to build into a pistol. I currently have a few spare generic "multi-cal" lowers, one of which i want to use for this build. I'd like some clarifications on some of the specifics on a few things:

1. how has "close proximity" been defined in case law?

2. how constructed does my lower have to be to be considered a pistol? can i just pair up a virgin "other" lower with that upper when i receive it? do i just need to screw in a pistol buffer tube? does it need to be built as a fully functioning lower (fcg, buffer kit, etc) to be considered a handgun?