IDENTIFY MY booster.

HELLO, guys

i think i have asked before but couldn't find any evidence that i had. I am looking to replace my brake booster and master cylinder on my 1968 plymouth barracuda (not original or original owner.) it has power steering - front disks rear drums - with a dodge 440- i am browsing around rockauto but shipping is killer so i want to shop around autozone and such but not sure what i am looking for.

my question is how many different ways is there to know what you are looking for when shopping for a booster cylinder package or what am i trying to identify? rear cracket? complete size? booster diameter? i am trying to find smallest size possible to have more roon under the hood.

thanks