Unfortunately, making a short bbl rifle isn't as fun as it used to be. The last poster indicated that you did not need to worry about the folding stock... unfortunately, you do. A SBR is not exempt from the "Crime Bill". If the SBR has never been assembled into an assault rifle with the evil features, it cannot be made into one now. Even with the $200 SBR tax stamp.
If you started with a preban rifle that already had the evil features, you'd be fine.
Granted, most folks will never be able to tell that the receiver you use was preban or not, but you would not be complying with the letter of the law if you built a SBR with a folding stock on a virgin receiver.