I had a new 1969 Chevy Nova SS ... with the 396/375 HP option and a 4 speed. It came with a 4.10 rear gear. I ran low 13's with it in stock ... original trim. Nothing done but jetting and timing changes. With headers, a Hurst shifter and more jetting and timing changes ... it went mid 12's. I later bought a 70 Chevelle SS 454 with the 450 HP LS - 6 option and it was a low 13 second car also ... with nothing but jetting and timing changes. headers and tuning got it to the mid 12's also. Both without ever removing the engine or having the heads off.
I raced at a NHRA track ... @ about 200 feet elevation. Traction was good ... and the air good.
The biggest competition came from other Chevrolets and the 440 6 pack Mopars.