I have a 390 carb. I've rebuilt it twice. I know it inside and out.
The top small port is for vac advance. Bottom small port (on the base) is for the trans vac line. The large port on the backside in your first pic is for the PCV. For your brake booster use a threaded port on the intake manifold. You'll have to buy a nipple to attach it.
On this pic you can see the vacuum line for the brake booster coming off just below the carb. The hose swoops down and goes into the booster in my case. This was on the OEM intake manifold but the Offy is similar (bung is at the rear of the manifold though). Look for a 3/8" NPT threaded hole. Probably is plugged now.
