No it is not illegal to finish and possess such guns. No, there is no limit to how many you can finish in a year, nor how many you can possess in total. No, there is no way to trace them, if you go at it right. The 1911 variant is very easy to finish. Check them out at KT Ordnance, The Tannery Shop, the "build it yourself" forums at AR-15.com, Dlask Firearms, DIY Firearms, and Home Gunsmthing forums. Yes, they do cost $50 more than a finished frame, but then nobody but you knows that you have it. That's easily worth $50, to quite a few million people in the US.