By the law you can't have the upper without the stamp. You can build the upper and give it to someone else that doesn't have a lower or a rifle that it will fit on unless that person has a Form 1 SBR stamp. You can always build it as a pistol and then make it an SBR once the stamp arrives. Also if you change things that don't jive with what the original Form 1 says you will be in violation. You can always build different uppers for the SBR as long as you keep the original upper.