It might be "the best thing to do to uncork the exhaust to let it breathe", but Chrysler usually did pretty decent with their cast iron exhaust manifolds, as to flow and power, unlike GM engines. In other words, not worth the effort for just another 20 horsepower at the flywheel.
The other thing is the heavier C-body platform will not be "a rocket" by any means. The lighter B-bodies better fit that bill, not to forget the even lighter A-body cars. With many options for the smaller platforms. Some of which might fit the C-body cars.
Does the car now have a 2.50" dual exhaust system under it now? If not, that would be where to put your emphasis, at the present time. Building for the future.
Going to build a 600 horsepower 360 in the future? Head for a 408 stroker with a 235@.050 cam, with some AFR or TrickFlow aluminum heads, and some Mahle piston/"MM"ring sets. With a much-upgraded TF99_ transmission. Then add a 3.21 SureGrip rear axle.
Respectfully,
CBODY67
