I have a '64 C20 with a 5 speed and am using a Clifford 4 barrel intake and Hedman headers. It works great. They make a significant difference. If you go to the Gallery you can see a picture of it and the engine at Ron and Kim Plender's '64. I believe those headers fit up through early '70's.
My 2 cents worth.

1964 Chevrolet C-20 Fleetside, Otis
1950 Chevy 1/2-Ton, Kirby
1950 Chevy 1/2-Ton Shortbed, Walter