Power Booster Size for BB

[stokers]

I currently have a BB in my '67 Camaro and want to replace the 11" power booster 'cause it really sits too close to the valve cover. My question is what size can I use and still be safe? Is a 7" booster too small? Should I get a booster that is single or dual diaphram booster? I'm not really sure what the benefit is for having a dual diaphram over a single. Maybe someone could enlighten me.

My brake set up right now is front and rear disc.

Since I am replacing the booster I was planinning on also replacing the proportion valve which now maybe I'm getting to the point I should consider a hydraboost. What do you all think?