The boss 302 heads are almost identical to the 1970 model 351 clev.4v heads except the boss 302 heads have a smaller combustion chamber. The biggest problem with the 351 clev 4v and the boss 302 heads are that the exhaust port had a hump in them restricting flow. The heads you have have fixed that by raising the exhaust port with the alum adapter. I don't whether they are Bob Gliddens ex heads or not. I'd say they are Yates heads, he's the one that started raising the exhaust port for better flow. I don't know of any small block cast iron heads that will even come close to touching the flow of the heads you have. I ran a 68 Mustang 20 years ago with a 351 clev 4v Yates heads. It ran 10.90s at 125mph with stock suspension. I still have 3 sets of 351clev 4v closed chamber heads and one set of the 302 Boss heads.
On the C9ZE6090C cast number, the 1st letter C means 1960s and the 2nd letter 9 is the year in the 60s the head was made. Your heads are 1969 Boss 302 head casting. Boss 302 heads were only made in 1969 and 1970. C9ZE and D0VE castings. The first Cleveland 4v heads came out in 1970 and were D0VE castings. D for 70s and 0 for 1970.