GN My 87 GN was swapped to vacuum - which booster to go with?

[paradigm]

My 87 GN was at some point swapped to vacuum, and the brake booster looks quite rusty. I am going to replace it with a new power brake booster.

Any suggestions on which to go with, or any generic brake booster from a 305 G body from Rock auto would suffice?

Also what is this grey harness below the brake booster? And the rectangle cutout in the firewall? Is that for the old powermaster system?

 

[Malibu795]

Looks similar to mine...
8" dual diaphragm off 85 MC..

The one I used was AZ# NM1926 (1-5/16 bore) and 8" dual diaphragm #54-71243

Thing to note... The MC bore that sets in the booster... There's two sizes and NOT interchange changeable

 

[Slowpoke]

The square hole is for the wiring harness bulk connector.. similar to this.. the gray part looks like the engine part off it