are yours old or new ?
do you want a higher lift cam then standard ?
because you usually buy springs to suit the cam .
I'd say std new 318 spring would take around 440 lift .
& thats if there new . springs loose tension ( just like chicks do
) after a fair bit of use .
paw performance automotive had some for about $40-50
Mancini racing has comp cams springs for $60
both of the above springs were good for .500" lift
just shop around on line & find the closest spring rate to your cam lift
just remember you need lifters as well .