What would the (approximate) horse power gain be (if any) with original Z28 exhaust manifolds over original stock small block exhaust manifolds?

I have looked at aftermarket exhaust headers for a long time & read a lot about them here on Team Camaro.

I have factory air conditioning.

No one seems to ever have a definite answer to putting headers on with factory air. I got to thinking maybe there would be an advantage to the original Z28 manifolds over the stock (non-high performance) manifolds. Then I wouldn’t have to worry about a/c box & power steering box clearance issues. I have an all original 275/327 with a/c. What thinkest thou?

I have always seen the stock Z cast iron manifolds the same as stock 350 cast iron manifolds. Now installing any type of performance type header on any first gen , with or without AC is a plus. Its just common sense. But headers alone will still leave you with the stock restrictive exhaust system . Ie remaining header pipes, transverse mufflers etc.