There is a lot momentum even when a truck that size is running without a payload. Bashing the Lexus that cut you off suddenly will not be a good way to end a pleasure cruise.
If the hydrovac was disconnected, I would suspect that it went south before that. They can be rebuilt, sometimes NAPA carries rebuilt units depending on available cores. At the risk of suggesting something besides bone stock, I would recommend a diaphragm type Midland brake booster like was used on '70s GM trucks. I can't give you a model # off the cuff, but they were common on motor homes, RVs, and one ton and up trucks. On split systems, one was used for front, one for the back. One of these would give good performance when replacing the hydrovac. You would need to fabricate a frame mount for the unit. If you wanted to go with a dual master cylinder, there is also a unit which has dual boosters offered by ECI hot rod brakes. They could give you more information on that.
If I find the model #, I'll post it here as I level my table top.
