Mad Dog offers headers for the B-body, both d-port and round port. Good stuff, Mad Dog. And it's true, with minor "alterations", first gen Firebird headers can "fit".
Most 62s I've CCd as"known virgin" come out to about 74 CCs. The statement about NOT using factory specs is RIGHT ON THE MONEY. Not just with Pontiac, but most American cars of the era, the published data is "nominal" and not necessarily accurate. The chambers MUST be CC'd to really KNOW what they are. Sometimes, you just gotta do the "leg work".
A standard bore 389 should be about "right" for 93 octane with 78-80 CCs in the chamber. The valve reliefs are about 6. Cutting a "dish" in the piston to enlarge the chamber is commonly done.
On that, the pre-67 performance heads were 19 degree valves, and the valve reliefs are close to the center of the piston. For '67, with a couple rare exceptions, the valve angle was changed to 12 degrees, moving the reliefs "out" closer to the bore. With cams under .470 " valve lift, this is no biggie. For more than that, there COULD be valve-to-piston interference. For rebuilders, this is the purpose of that horrid looking 8-relief piston.