Just went through myself. There are bushings in the manifold where the shaft goes through. If the bushing stays put, tap it 1/8" NPT. If the bushing comes out, you can drill it 7/16" and tap it 1/4" NPT.
"The DD7 header set is designed to fit small block Chrysler (318-360) V8 engines in Dodge, Chrysler, Plymouth applications, such as: Demon, Duster, Dart Barracuda, and other Chyrsler A-Body chassis." They are listed for $345