Thanks for all the help guys. I received a Cardone 10.75" dual-diaphragm vacuum booster for a '90 Caprice yesterday, and managed to get it fitted (just barely) today. The bottom right edge of the booster is just barely touching the top of the wheel well covering (circled in yellow), so I don't think I could go any bigger without some modification.
Plus, none of the various DIYs mentions some of the smaller, yet time consuming, issues one might face doing this job.
In addition, I needed to temporarily remove then reroute some of the grounding against the firewall (green circle), and the wiring harness for the bundle of cables going to the relays on the driver's side fender (in purple) had to be removed/discarded to push the wiring loom down far enough to get the booster in. Plus the vacuum line to the cruise control (red) will have to be rerouted a bit, and the various line that were right above the Power Master had to be moved up more. Same goes for the throttle cable, which had just enough slack to get the booster under it (although it might be better under).
Then there was this little bastard lock washer.
View media item 463This holds on a wiring harness inside the car on the lower left stud of the Power Master adapter place as it comes though the firewall. It didn't want to screw off, so it required the coaxing of a foot-long, very skinny, common screwdriver to pry up the tabs, and a pair of long needle nose pliers with a 90* bend at the end.
I am NOT looking forward to removing the pedal. Any advice is appreciated.
What's also odd is that most vacuum conversions state that the booster arm needs to be connected
ABOVE the existing PM point on the brake pedal. However, the new booster arm seems aligned about
a 1/2 inch below the existing point. Hmmm...