It's possible to bring in your lower, but whether or not the guys in the retail storefront could accommodate the "fitting" request is up to whether the circumstances are right, i.e. we have several uppers for you to choose from, whether its busy, etc. While we wouldn't overtly make this a policy, to allow a customer to choose their upper while other customers wait, we DO try our best to help you get what you want, and it's possible one of the staff would be happy to help you out on this when you are in the shop. We always want to go above and beyond the basics to satisfy our customers, but we also need to remain practical, so please understand it may not be possible to do so at all times.
As for the timeline on receiving the uppers, things are so ridiculously busy, the only surefire method of knowing when we receive them in stock is by signing up for email notification on the specific pages of the items you want.
What type of lower do you have? Is there a matching brand upper we stock? Or are you building the gun with one off pieces? Many times, it's hard to find a good match, as batches of receivers sometimes are so close in dimension that they don't vary much. Do you have a reason to believe that your lower is spec'd out of tolerance or are you simply trying to get the tightest fit possible?