I have a 350 poncho that I have a set of closed chambered heads for. It runs pretty good now but I am considering doing something similsr to your Idea. I have some closed chambered heads (i havent CC ed them yet) that I was looking to run but they would put the compression ratio over 10.25 / 1.00 to much for pump gas. I am toying with having them cnc machined to a larger chamber size but with a specific shape in mind for flame propagation to increase burn speed with out having to much compression for pump gas. There is plenty of metal to remove for changing the shape to a heart or kidney shaped chamber and to also unshroud the valves wich I have heard is a problem with the 350 pontiac due to the bore size. ( Small). This creates a problem when going to larger valves as it makes the problem worse and then you have to chamfer the top of the cylinder to gain clearance. (Difficult and tedious)
With the 326 heads that would not pose a problem as the valves are smaller and would be less shrouded by the cylinder and the chamber.
Some port work may be beneficial . I would not do any to the ports them selves ( except for a deburr of the entry and runner) but would focus on opening up the "Bowl" behind the valve as to increase the airflow in that area.
That would probably gain more than bigger valves anyway, In my opinion.
One thing I have found also is pontiac exhaust ports are smaller than intake ports, at least more so than an average engine.Pontiac uses a dual pattern camshaft design ,the exhaust duration is longer than the intake duration. Check out camshafts for pontiacs, you will see, and that is why.
CNC machining can be pricey you can dump a buttload of cash in a set of heads, probably as much as a set of aluminum performance heads from what I hear, so spend some time thinking it over.You might want to read up on porting and C C ing chambers and such so you can do it yourself and save bucks. Maybe get a cracked head at a machine shop to practice on a little first and get a known race guy to critique it for you to get the Idea and a feel for whats happening.
By the way 65 heads and up will fit ,64 and back have a different bolt pattern for the intake,and I think that 72 and up may be different also. The difference in the 64 back is the bolt pattern, 72 and up is a different exhaust crossover and would be a big exhaust leak on a 71 and back head. I think that is an accurate quote.check that out also, it could be a problem for you as well.