Most of us don't consider using the stock log-style exhaust manifolds for high performance. A set of headers or the factory HO/Ram Air manifolds are usually used in place of the more restrictive standard log exhaust manifolds. However on some of the milder closer to stock engine builds the log manifolds can work just fine.
Here's a little modification that can be done to the log exhaust manifolds to help improve them. If you have access to some high-speed grinding tools used for head porting you can work on opening up the outlets. You can go from the stock 1.850" opening which is same basic size as the inside diameter of a 2" exhaust pipe, to 2.150" which pretty much matches the I.D. of a 2-1/4" exhaust pipe.
Shown below are a pair of right side 4-bolt 1969 log manifolds, casting # 9796992-1 date coded E129 and J078. The upper manifold in the picture has been opened up to 2.150", the lower manifold has the stock 1.850" opening:
The early-style right side manifolds have a larger opening than the left side ones, the right side was bored out to around 2.050" to accomodate the thermostatic flapper valve that diverts exhaust flow through the intake manifold crossover to activate the choke during engine warmup. In the 1968 model year the flapper valve was eliminated from the right side manifold if I'm not mistaken, but the manifold outlet was still bored out and was larger than the left side manifold outlet. By 1969 the right side manifold no longer had the provison for the flapper valve, the outlet casting was shortened and the opening size matched the smaller left side outlet.
Here's a look at the center manifold inlets at the head flange on a 1969 9796992-1 (at the right) and a 1964 545467 equipped with a thermostatic valve. Note the wider casting and longer bolt bosses on the earlier manifold coming off of the head flange:
A look at the inside, note the smoother inlet on the early manifold and the slightly more restrictive opening on the later inlet with the bolt clearance notches cast in:
It looks like using some of the earlier log manifolds might offer a little better exhaust flow. Removing the flap and shaft and plugging the shaft holes on the earlier manifolds would be the way to go.
