Re: My 352 heads
I'm by no means an expert on FE heads, however, the main constriction on the FE exhaust manifolds/heads is the angle of the exhaust ports. The back half of the engines exhaust ports is angled towards the front, therefore if your running the traditional "log" type manifolds, the exhaust pulses always fight each other. Depending on what the end result of your engine build is, this could be the constricting point of the build. The 6% increase your referring to is actually a little skewed. You must remember that this increase is during lift thus you have a conical type shape (surface area of cone) that has to take into account the angle of the seats/valves when doing the % increase. Here again, size of valves is directly related to camshaft you will be running and what the end result of the engine build is, i.e. operating RPM. There is always trade offs. When picking headers (if there are more the 1 option), the same criteria stands. Rather it be shorties, full length, tri-y's, etc.
Luckily you dont have the thermactor type heads that have the bumps in the exhaust port. One more thing to keep in mind, the exhaust port locations changed with the emission styled heads, post 67 (if memory serves). Emission style heads raised the exhaust ports in the heads approximately 1/4", thus if/when looking at headers, keep this in mind. Just because it has the correct bolt pattern, doesn't mean the ports will neccessarily line up. This makes for sealing the exhaust flange nearly impossible.
If you decide to port, best advice is to get a junked FE head to experiment with. Jo-Anns Fabrics (or similar craft store) offer a putty that you can mix together pour into the intake and exhaust port and make a removable/flexible mold to get a visual idea of what exactly your dealing with. Then you can have an idea of where the main constricting points in the runners will be.