Well when you say production, do you mean readily available or are you open to more rare and thus expensive alternatives.
Best flowing Pontiac head ever is the RAM AIR V head, but good luck ever finding any not to mention exhaust and intake manifolds. These also were only available over the parts counter at dealerships and were never installed on a production line car.
Best flowing cast iron head is the 73 and 74 SD455 heads, cast number 16/x, but again, you'll pay a fortune for these. Any round port Ram Air II/IV or HO head will outflow all other D-port heads, but again are going to be very expensive and not worth the money when compared to an aftermarket aluminum head. RAII and IV are going to have small combustion chambers and put your CR over 13:1 unless you dish the pistons. The later '71 and '72 HO heads have large combustion chambers and will drop your CR down to around 8.5:1. See the following link for the cast numbers
http://www.wallaceracing.com/head1.htm.
Now for real options using factory cast iron heads that are readily available and affordable, I would look at the '71 and '72 400 4bbl heads cast number 96 and 7k3, followed by maybe the 6x-4. Any of these would make a good place to start.
But in all honesty, unless you know someone that knows how to port the heads and they are doing it for free or little money, I would move right past the factory Pontiac heads and go with the Edelbrock or Butler heads. You can get either D-port or Round Port designs. By the time you buy a decent core, have all the machine work performed, plus replace components the aftermarket heads will more than likely outperform them right out of the box. With porting, the aftermarket heads will far outperform pretty much any factory head.
