Best bet is to go with a HydroBoost from an aftermarket supplier.
Spend the money one a current unit that is gauranteed to work . Don't gamble on a re-man 84-85 unit (which have a high failure rate).
The newer units feature an accumulator & are best as they give an additional 2-3 stabs at the brakes if you loose your PS pump.
HydraTech, Talon Hydraulics (now Vintage Speed Reaper), CPB, etc. are just a few names for you.
Most of these are current Bosch HydroBoost units (who own the HydroBoost trademark name) , unlike the 40+ year old technology used on the 84-85 Buick HydroBoost systems.
Only downside is the price...you should expect to pay just under $1K, but includes SS braided lines and AN fittings.
Get in touch with Intercooled88s on the forums, i was having trouble finding them also and got mines from him. I did however order 1 from both Rock Auto and Advanced Auto just recently but both of them showed up without the firewall mounting bracket and made no mention of this in the description. Aftermarket is the best option if it fits into your budget.