The A and the C were the same internally, except for the camshaft. I am not sure but I believe the heads were the same. The C used a dual-plane 2V manifold, the A a 4V. The K code was basically the same block as an A code, with some of the internal components beefed up, so it could handle higher RPMs to allow higher performance top end components like free-flow induction heads exclusive to the HiPo. The K utilized a solid lifter cam where the A and C code engines hydraulic cam. A "true' GT only has an A or K code 289. I must confess, even though my 289 is bad to the bone now, it began life as a C.
1966 GT Convertible
1969 Sportsroof (which is For Sale, by the way)