I doubt the tuning is the main culprit here. Get some 1.6 RRs on the car (the full roller rocker should give you a mph or 2). What were the conditions like when u ran the car?

I ran a 12.6 @ 110 on 100% stock tune in my '97 with a smaller cam and stock heads (ran 12.4 @ 111 w/the GM 847 and a half *** tune) and the conditions here are never great. With LTs, 1.6 rrs, and better conditions I have no doubt you'll trap ~ 108 which is about right for an auto w/out an agressive stall/gear.

 

Is the engine making any funny noises like 'clattering'?

Maybe you didn't adjust your rocker arms correctly? You might have them too tight or too loose, to tight would cause the valve to never close all the way and too loose would cause the valve not to open all the way.

Remember, a half turn down from the top of the plunger seat.

-Dave C. '97 Z28