How fast have you gone with exhaust manifolds?
This is my 65 Dart, decent 383, 4.10's, 4-speed and 7-inch slicks that were hard as nails.
I don't know what the manifolds were from, I went through a pile until I found two that would fit in the car. The car ran mid 12's @ 110 to 112, video here. Using the weight to calculate HP, the manifold cost 60 HP. This engine with headers in an E Body which weighed 300 pounds more than the Dart went about 3 MPH faster (but was a whole lot quicker since it could move out of the hole without spinning the tires.)
This is my 65 Dart, decent 383, 4.10's, 4-speed and 7-inch slicks that were hard as nails.
I don't know what the manifolds were from, I went through a pile until I found two that would fit in the car. The car ran mid 12's @ 110 to 112, video here. Using the weight to calculate HP, the manifold cost 60 HP. This engine with headers in an E Body which weighed 300 pounds more than the Dart went about 3 MPH faster (but was a whole lot quicker since it could move out of the hole without spinning the tires.)