I want to sbr mr ar for an 8.5" 300 blackout upper. If I have a stripped lower that has never been a rifle, can I order the upper while waiting on the stamp?  Or do I need a pistol buffer tube on it?  I have no extra stocks laying around, so it wouldn't be constructive intent, right?  I don't really want a pistol lower right now, because I want to spend my money elsewhere, but I want all the parts ready when the stamp comes in.

Could I have the upper delivered to work, lock it in the filing cabinet and not bring it home until I get approved?  This seems iffy, but would save the price of the lower.

I know I could have someone else order the upper and store it until I can take possession, but I don't want to go that route.

My understanding is that you can't have a combination of parts that could be used to assemble an illegal or unregistered configuration.

Example, if I have two complete AR-15 rifles, a stripped lower and a short barreled upper the only possible combination of parts would be assembling the the short barreled upper on a non-sbr lower.  Since the stripped lower can't be assembled into a firearm technically.

If I make the stripped lower into a pistol, I now have a legal purpose for the short barreled upper.

Most people go the pistol route and when the form comes back throw on a riffle buffer tube and stock. This is the route I would go personally.

With how cheap lowers are right now, buy a $40 Anderson lower, a pistol build kit from somewhere and enjoy your .300 blackout until the stamp comes back on your lower.

Edit: Someone may have more knowledge than I and I'm not a lawyer so the above isn't legal advice.

FIFY 

"Constructive intent" isn't a thing. Constructive possession is a legal concept. If ATF wants to charge you, they charge you with possession, then they prove constructive possession (or actual, if that was the case).

Nothing prohibits you from ordering a short upper at any time. What if it's for a pistol? The seller doesn't know/care.
You do not need a pistol buffer tube. You could assemble your virgin stripped lower as a pistol using a standard carbine receiver extension, just do not put a stock on it until your Form 1 comes back approved.

Once you make a pistol, from what previously was never a rifle, it can switch between rifle and pistol afterwards.

So since I originally built my lower without a stock and just a bare buffer tube, I can remove the stock, make sure it isn't in my possession and put it back to a pistol lower?

To complete the initial pistol assembly, it would've needed a <16" barreled upper as well.
But yes. 

The stock may remain in your possession; you just need some legal use for that stock. As a spare stock for another carbine, for instance.

I got a kak pistol buffer tube from Rainier Arms on a sale for $20 and it uses a carbine spring and buffer.

All my lowers are pistols 1st and it only cost me $20.....shoot it all you want as a pistol and when the stamp comes back....Un pistol it

Best part is if it runs as a pistol it will as a rifle too, being all running parts are unchanged.

If you can put the short upper together in a legal configuration, you should be safe.  I have a pistol lower, built from scratch as a pistol lower.  I've never taken it to the range.