My 273 runs low 12s in my 65 Cuda, with a .400 lift cam, 318 heads, stock intake and carb, but there are so many other items it takes to make it go that quick, transmission is a big one.
Cam duration, Compression ratio, what's been done to the cylinder heads, carburetor modifications, transmission modifications, gear ratio, stall converter, type of headers, ignition system.