Brake booster question for you guys.

I have a 1966 A100 with manual brakes. wondering if I can just add a power brake booster to it so I can get better braking? The master cylinder is under the floor and not much room for a big one, thinking maybe a 7" one might fit in there. I see they sell single and double diaphragm. Which would work better for my app? Thanks for any info.