I have an AR Pistol with a 7.5" barrel.  It is a fun gun for the outdoor range.  I just purchased a suppressor and am waiting.  Now, I want to build a SBR.  I was thinking of 10.5" and using the suppressor on it.  I plan to build it as a left hand gun.  I have another lefty and I like it.  I do shoot lefty.  I was thinking that I could also use the 7.5" pistol upper on the new SBR lower.  My suppressor is rated down to the 7.5".  So the plan is a 10.5" lefty and a 7.5 righty upper.
My question is what barrel length and overall length do I put on my Form 1 ?   I would hate to designate the 10.5" on the Form 1  and find that I like the shorter version.

put the shortest possible & build what you want

Put on the form how you plan to first build the SBR.  If you're plan was to start it with the 10.5" barrel, put that on the form.  Once the form is approved and you build it with that 10.5" barrel, the SBR will then exists as the form was submitted, and you are able to then freely, at will, and without further tax payment or permission from the ATF, re-configure that SBR with any length barrel you want, and back again, etc.

Installing the 10.5" barrel first is my plan.  Do I specify the length overall for the 10.5" ?  I think I calculate 29.5" for the 10.5" barrel configuration.  It just seems odd to me that going to a 26.5" with a 7.5" barrel would be of more concern to those that appear to be concerned about these things.  To me, I would be concerned with the most concealable configuration, i.e., the shortest it could be.  I guess I will never understand government regs and requirements.

Nobody cares
List what you're building first.
Consider notifying ATF of permanent changes. Swapping uppers is not permanent