Just curios...If I plan on making the OAL of a short barrel 16" BEFORE mounting it to an upper receiver, is it legal for me to purchase one without an SBR permit?

If it's 16", it's not an SBR, no permit required.

You cannot be in possession of both an AR receiver AND an AR barrel of less than 16" length.  If you possess an AR receiver and buy an SBR barrel wanting to permanently add length to reach 16", you'd be in violation and vulnerable for the whole time it takes to do this.  Either buy the barrel already modded or get the receiver out of your possession while you have the barrel in SBR length.

The violation here, BTW, is "constructive possession,"  it means you have an illegal weapon by just having all the individual uninstalled parts under your control.

or an other way would be to buy a stripped lower reciever so that you can claim that the short barrel is for building a pistol.  However you can't have any stocks or stock kits lying around with this option.  You could always just buy a complete pistol lower so that you aren't in voilation of the 'constructive posession' thing.  Although it's a somewhat stupid law considering that you can make any AR full auto with some paperclips...

I have a lower with the parts in, but haven't attached the magpul stock yet.  I don't have an upper reciever yet, so if I just bought the barrel, then sent it out to be modded before buying the upper I would have all of the part to make a comple weapon anyway.  I guess I could always leave the rear stock at my friends house a claim I am going to build a pistol if forgoing the upper won't work.