55.2-59 Brake booster opinions

[tbrown5079]

I built a sweet 56 cameo with a 4.8 LS engine and a GM A body front clip. Lots of other upgrades. Happy with everything but the brakes. I used a firewall mounted kit with a 7" power booster and swing pedal bracket for under the dash from a popular Chevy truck supplier. I had the booster replaced once (lifetime warranty). The engine is stock with low miles and has 20" of vacuum at idle. The brakes work OK, but I just can't really set it down like I may have to sometime. They have 8 and 9 inch boosters with single and dual diaphragm that would fit my kit. I have room for a larger booster. The mounting is 4 bolt 3 3/8". Is bigger better and why do they use dual diaphragm now instead of single like they did before? Your thoughts? Suggestions? Tbrown