Changing your wheels to lightweight 15" weld rims also helps to reduce unsprung weight.
12's are possible NA with 3.73's, but you really have to start to reduce weight. I ran a 13.2 @ 104 mph NA with full interior, 15" welds wrapped in Nittos, and I still weighed in (with me in the car) at 3400 lbs.
Im pretty close and i have no suspension work. I think if i had some work done i could hit 12.99. Its a full bolt on car with 3.73's and ive ran 13.20 no weight reduction and it was on nittos. I think if i got rid of the seats got suspension and got full slicks i could do it.
This guy I know runs in the 12's consistantly. He's a damn good driver though too.
Here's his sig from another forum. I see him do it all the time. Oh, he's running on stock, cut springs to boot.