WhalePirot,
What you are referring to is the thermostatically controlled air cleaner heater duct. I'm not sure about '76 vettes, but '75s did not have this feature. The heat riser that I am talking about is between the right hand exhaust manifold and the exhaust head pipe. Same location as a normal heat riser, but it is controlled by a vacuum actuator that bolts to the right hand side exhaust manifold. Vacuum is ported from the carburetor, through a TVS (thermostatic vacuum switch) which is located in the thermostat housing. When the engine is cold, vacuum is applied to the actuator which closes the exhaust heat riser which in turn forces the right hand bank of cylinders to exhaust through a port in the intake manifold. In passing through the intake, the exhaust gases pass under the carburetor in order to preheat the carb. The exhaust then passes through the left hand cylinder head and out the left hand exhaust manifold. When the engine hits 170 degrees or so, the TVS closes, depriving the EFE vacuum actuator which in turn opens the exhaust heat riser, allowing exhaust gases from the right hand side to flow normally.
This was done for two reasons. #1, it improves drivability when the engine is cold and #2, it shortens the warm up time. Since the warm up time is shortened, the amount of time that the engine is running in a richened state (and the resulting pollution) is reduced.
So, here's the problem . . . there is no practical way to incorporate the EFE heat riser into long tube headers. For that matter, I'm not sure there's a practical way to incorporate any heat riser into long tube headers. This is the reason why the manufacturers have not developed CARB certified headers for these years of vettes.
Now, if a guy wanted to be really enterprising, he could probably figure a way to use shorty headers and get the EFE controlled heat riser to mate up. I actually have a set of shorties that I goofed around with for this purpose. However, the heat riser ended up sitting low enough that it would have taken some real pipe bending creativity in order to hook up the head pipes. And even at that . . . bottom line . . . there would be no CARB certification and that is what California lives and dies by . . .
So . . . three years and counting . . .