Our strip is at 6000 feet here, so my low 14s here are low 13s at sea level, so I've been told. I think 4.11 gears would get me to high 12s at SL.
C heads require plugging some holes, drilling some in the block and a special intake manifold, basically.
10 years ago, they were better than most aftermarket heads, but not nowadays...

 

did it this mornin and it was super straight forward..took me hour and a half..most time spent on nuts and bolts..great ride... forgot what it felt like to acually have shocks