I switched from 9" Single Diaphragm to a 7" Dual Diaphragm to gain more valve cover to booster clearance. 9" only had 1/4 inch at the most. The 9" I had was a repo '67 Vette style Booster & Master Cylinder unit that worked well.

Switching has a few problems that I uncovered the main being that 7" or 8" either single or dual diaphragm DO NOT have pressed/riveted bracket. I searched for quite a while and finally found a source for 63-67 Corvette booster brackets. Once I had the brackets, booster and master cylinder it appeared that it was just a simple bolt on just like the 9". Not quite. The additional length of the booster (3.5" vs. 5.5") caused the MC to touch the hood. I suppose you could go with a 7" or 8" single diaphragm but they only provide 950 psi with a 1" bore MC. The '67 style 9" single has 1100 psi. The 7" & 8" dual both provide 1200 psi. Seeing how I already had a chrome 7" dual it was time for a bracket/MC angle adjustment  J

What I did was put a 5/8 inch spacers behind the bracket at the top which pushed the front of the MC done far enough for adequate hood clearance. I used 1/2" thick wall black pipe for the spacers. One other problem I had was getting the correct length on the pedal rod. If I had known up front I would have ordered a pedal rod extension but I went to my local Autozone and got shock stud extenders. The difference in the two the best I can tell is the pedal extenders have many more threads. So I had to used a 3/8-24 die to add threads to the shock extender. Now I had plenty of adjustment.

After job was complete including bleeding brakes I now have LOTS of clearance and would be no problem install extra tall valve covers even with spacers J  

I found the brackets (Part #6466-7) at Pirate Jacks www.piratejack.net they also have Boosters/MC/extenders/etc. but you can also find everything except the brackets many places on the net.

Hope this will give you ideas and possible help.