Can someone point me to a resource where I can confirm the minimum overall length of a rifle and the minimum barrel length (without ATF paperwork)?

I'm pretty confident that the minimum length of the barrel has to be 16".  However, I thought that the minimum overall length of the rifle had to be 30", but I've heard two people say that it has to be 26", so I'm wanting a little confirmation.  It's probably irrelevant to an AR, since with a 16" barrel, it's about 30" with a collapsed stock, but it's good information to have.

Your correct the barrel must be 16", though you'll find most to be about 16 1/4" just to be on the safe side.  Minimum overall length is 26", under that and you'll have to go the NFA route.  I have a registered AR SBR with an 11.5" bbl. with collaspable stock and it is 28 1/2" with the stock closed.  If you want to look the laws up your self try www.atf.treas.gov be aware however that state laws may vary.

Note some states have laws which are more restrictive than the Federal rule.

Some states (like CA and MI) have a longer minimum length; also IIRC the CA law requires the measurment be made while the stock is folded/collapsed - while the federal law measures when the length is open  (Otherwise you'd not be able to buy M1 carbines with the wire paratrooper stocks).

Here is some info I ran acrosswww.quarterbore.com/nfa/sbr-aow-pistol.html

If a person were to buy a new lower , can it be registerd at the time of transfer as a SBR?

How would this apply to 80% castings that dont require a transfer?

80% lowers cannot be transferred after you make it into a lower.  To the best of my knowledge, it can be registered with the ATF as a SBR.