XXL, are the headers short or long length?
The only knowledge recommendation I have seen for tying the two collector pipes (three cylinders each) together for a six cylinger engines has been to empty each pipe directly into a common resonator, muffler, or catalytic converter. The larger volume vessel minimizes the interaction between the two pipes as seen at the exhaust valve ports.
An inline six using a single non-header exhaust manifold experiences some exhaust port flow interference due to an exhaust valve opening every 120 crank degrees, more so than on a straight four cylinder engine with an exhaust porting opening every 180 degrees. So using a shorty header, or one like Tom Landgon's Stove Bolt Cast Iron Headers would be beneficial.
I am considering an exhaust arrangement similar to yours, headers into a single exhaust for a truck application. So far, for a dual in/single out muffler I have located the following:
Flowmaster Model 524703, 70 Series Big Block, 2.25" x 2 in/ 3" Out
Dynamax Model 21308 MegaFlow RV
Both mufflers have large volumes, which does raise the question of its effect on performance.
Didn't the later 292 truck engines used a split cast iron manifold (3 cylinders each)? Does anyone know the exhaust system arrangement for the vehicles using these engines? I doubt they used a dual exhaust.
