Your original factory manifolds will work fine with dual exhaust. But you can use the factory HO (high output) long branch manifolds as seen in the previous post. You will also have to get the oil filter adapter. They will really wake your 389 up. These manifolds have been reproduced and are available @www.amesperf.com.
67-69 Firebird headers MAY fit your car with little modification.
My suggestion is to use the factory HO manifolds. They fit and work well. I own many mid 60'* Pontiacs.