OldIHC

I don’t quite have my engine back together enough to start and test it myself, I was wondering what kind of vacuum pressure should I be expecting. Is there enough vacuum to run a vacuum assisted break booster?

Terrell

I am not sure, but, I think it would not be unreasonable to expect there to be enough negative pressure to operate a vacuum booster from a manifold connection.
There are several trustworthy suppliers on the internet. SS Brakes is one.

Large heavy duty trucks from the 40's and 50's ran hydrovac brake boosters from smaller capacity engines than that. I had a 40's 5 ton Army truck with a 216 cu in motor with a factory hydrovac , the system worked quite well!

Engine size doesn't matter, the vacuum booster stores vacuum, the atmospheric pressure is what supplies the "boost".
Small, low power engines that spend all their time "wide open" often have a vacuum storage tank, just like an air braked truck has an air tank.
There is a regulation for DESIGNERS of brake systems that there must be enough stored boost to stop the loaded vehicle in X number of feet and Y speed with the engine dead. Unless you are designing your own system, you need not worry about what the regulation requirements are, the maker of the vehicle has to do that. If you are changing a designed system, you better know that you meet or exceed the requirements for the size and weight of the vehicle you are modifying

If you screw this up and there is an injury or death that is your fault, you may have just lost everything. Your insurance company will walk away on you until you are able to positively demonstrate your modifications meet a standard. You insurance policy requires your truck to be roadworthy and your insurance rate and guarantee is based on this. Any changes that can be demonstrated to have contributed will cause your insurance to be withdrawn. It is all in the fine print. If you are not a certified mechanic, it will be very difficult to be held as an expert.
I get all of my modified vehicles inspected by recognised expert. This take the liability off me and onto the inspector. These inspectors are tough and the details matter. I was rejected over a small loose 5/16" fastener. The fastener was holding a brake line support clamp. A turn of the bolt and my R120 was passed.

I added a remote vacuum booster from a 59 Ferd 1 ton to my L-112 panel, the factory 220 provided plenty vacuum for it to function,

Let me put this another way. Engine size is immaterial to the level of vacuum ( pressure) . That is determined by atmospheric pressure and how much leakage the engine has by the rings and the closed throttle. the smallest lawn mower engine and the biggest RED Diamond 6 can pull the same vacuum. How fast it can do that depends on displacement and speed of the engine. How "low" it pulls is not something that is determined by engine displacement.
It is why when small displacement engines use a vacuum reservoir, to make up for the slower pumping time of a smaller displacement engine, it doesn't effect how low it will pull. A single cyl engine would pull enough vacuum to run a booster, it would take more time to evacuate the booster than a 6 cyl engine.
The way booster work, THEY DO NOT rely on the pump down time of the engine when the brake pedal is depressed, rather it pumps down when the engine is started, and maintains the vacuum inside the booster. When the brake pedal is depressed, one chamber in the booster is opened to atmosphere, and the "higher pressure in that chamber, when compared to the vacuum in the other chamber make is "boost" in mechanical effort.
By storing "extra" vacuum in a reservoir, to quickly replenish the vacuum in the 1st chamber of the booster when the brake are released, the time required to cycle the booster when brakes are applied repeatable times reduced.
Since a check valve is used, the system stores the highest vacuum (lowest pressure) that the intake has ever seen since the last application of the brakes.

My BG265 routinely pulls 21-23” of Hg at idle for what it’s worth.

That does depend on altitude, I live at 7,700' in elevation and my 220 pulls about 15 which is a good number for a healthy motor at my location.

readings at idle aren't really the point. Vacuum reading will go much higher when the throttle is closed at say 2800 rpm and THAT is what gets stored in the booster ( behind the check valve).