I have the Thumper heads (stock valve sizes and heavy duty springs) with a Comp Cams XE270HR-14 cam and it's gotten this 3800+ pound pig down the track pretty quick for what it is.
When I first took the car to the track and it ran slower than the old motor's times, I was convinced I needed better heads and would tell all the guys that I think I'm gonna go get some AFR 165s.
As the car sits now, it's 1 second and 9mph quicker than the old motor. I gotta believe that the weight difference between my car and a Mustang would put a Stang in the high 12's with this set-up.
Good luck with the new build, man. Maybe I'll catch ya at E-Town next season.