I have the dual cast manifolds on my 81 F250 and I like them. Things to consider:
- they use 2.25" pipe
- they have no heat provision for the automatic choke. I converted to manual.
- they have no manifold heat provision. If you look in tech tips, you'll find a nice photo essay on building adapters to provide hot water heat to the intake manifold.
- they have no warm-air provision for the air cleaner. I bought a heavy aluminum foil basting pan at the supermarket, wrapped it around the front manifold as best as I could, and slit a hole in it for the heat riser tube. It works.