The best manifold for a 318 is one w 318 sized ports....The only one ever made was by Edelbrock in the old days....model D4B it has GREAT throttle response and is the BEST manifold I've used on a 318 bar none.

Take care,
Rick
68 Coronet R/T 440 & 68 Charger 528 Hemi,and 5 Challengers! 6 cyl, 318, 360, 383, 451