Thanks for stepping in and clarifying Beretta686, you pretty much nailed what I've got going on.
The only spare lower I had sitting around was stripped, but I had all the parts to assemble it as a pistol (including the slick-sided pistol buffer tube). Didn't have a stock or 6-position tube. However, James made a good point about the constructive possession thing so I just slapped it together as a pistol and all the rifle building doors remain open in the future.
Just to make it totally clear, I know better than to assemble any sort of Title II firearm- even if in the privacy of my own home. Prison sounds like no fun.
Brian, that's a great idea that hadn't crossed my mind. I think ultimately what I'll do will be to send off the Form 1 and take the future as it comes- whether it involves a good NFA lawyer or a sad letter to the ATF.