So, My quicken willmaker software is in the mail, and I plan to buy a stripped lower receiver at the Fun show this weekend that will serve as the base for my first SBR.  When I put my trust together, I should just put this receiver into it and then fill out my paperwork, get approved, get the receiver engraved, and change the form of the receiver to a SBR in the list of the trust assets or how does that work???

Also, can I put my sbr upper together now as long as I don't mount it to a lower???
I ask, because how will I know how to fill out certain boxes such as overall lenth and so on.  

Buy the stripped lower
Create your trust and assign the stipped lower to the trust, list mfg and serial number.
Send the receiver off to be engraved
Once you get the receiver back in one piece and all is well
Submit your form 1

If you have an SBR upper in your possession and you have an AR15 already, they could say you have intent.  As far as filling out the boxes, since you can't just use <16 and <26 anymore you have to put something.  I used dimensions from Colt to do this.  For instance they list the 11.5" commando as 30" OAL.  I will build a 10.5" rifle so I put 29" in the box.  All you can do is make a guestimate without building an illegal rifle to measure before hand.  If you are way off, you can always amend the form 1 with a letter to the atf once the rifle is complete.