carburator spacer

i'm going to get a carburator spacer and i am curious on if it makes a difference what kind and what material it is...i know there are variables and i'll do my best to explain my engine, i can answer any of your questions that will better help you answer mine

Engine size: 402
Carburator:Holley 750 Vacuum Secondary Electric Choke
Exhaust: Headers then straightpiped to glasspacks
Intake: Edelbrock Air Gap
NO BLOWER

all suggestions are greatly appriciated and i will answer any other questions you have...