Re: c6ae-r or c6ae-u heads
The U heads are 390 heads possibly GT. Good heads, but not if your going for "big" performance. They have the smaller intake ports made for velocity...flowing alot of air through a small port, where low end torque is achieved.
The R heads are a (352) or 390 and 390GT even possibly 428, tall ports for more air flow. Install a set of CJ (2.09/1.65)valves in the R heads and you'll have a nice set.
The 14 bolt patern was designed for the type of exhaust manifold bolt configuration used.