If it were mine and I absolutely had to use cast manifolds, I'd make some type of step-up adapter from 2" to at least 2-1/2" as near the manifolds as possible(even if it just were welding larger pipes onto smaller pipes, say welding 2-1/4" dia. two inches long onto 2"pipe connected to manifold, then welding 2-1/2" pipe onto the 2-1/4" "adapters" kind of like stepped headers).
Then you could use 2-1/2" exhausts/mufflers for more flow. Even if the exhaust is coming out of the restricked manifolds small dia. opening, it will still expand enough to make use of the larger exhaust/mufflers for more power/torque and reduce backpressure IMO.
Someone probably makes an adapter for this already, and have for many years, expanded pipe or whatever, but I don't know the details. Google it.
Overlook the spelling.
PS I know you said in another post that you had tried different types of headers. Did you ever try Tri-Y's instead of 4 into 1 types? These Thorleys have a 2-1/2" collector.
Last edited by ssmonty; 12-26-2012 at 07:51 PM.