building 273..or not

My 273 setup -
4 speed man.
port n polished heads
dual exhaust w/ h-pipe
Lunati Cam .515/.525
Holley double pumper 600cfm
8 3/4 sure grip 4.10 - 26" tire
stock exh. manifold but looking to get headers to work witha 4spd n pwr steering

Never ran her at the track...power band is from 4000 - 6000rpm
Compared to other cars I had it feels like a high 13 sec. car if I ever learn how to launch a stick properly... About 12mpg cruisin' hard