All I'm saying is that the lower is the firearm, so you can set up the transfer for just that and then have the seller give/sell you the upper separately. The Form 4 will have a barrel length on it, but that doesn't mean the upper has to be with it. You might have to pay for two stamps depending on how you set up the transaction, but I'm assuming you're going to go through a FFL to hold the item for you. You may not legally have to do that, but it can be a good idea in case the seller dies/moves/whatever.

I'll defer to the FFLs here on the intricacies of moving a Form 1 to a Form 4 (or a Form 4 to a Form 4), as I've never understood what happens that's different when something goes to a dealer and then goes on a Form 3.