Leaking Venturi Boosters / Edelbrock Performer 1404

Where is the Flapper Valve located on the exhaust manifold?

Passenger side at the exit of the manifold. It should be able to be twisted by hand and a external spring should be seen and free.

Exhaust Heat Crossover Passage; Car has an Edelbrock intake manifold so would that still apply?

The head, is stock, has this passage. The intake manifolds from Edelbrock will have it on the Performer, LD4B, LD340, RPM, Torker(s)
The race intakes will not have them.