If you rebuild every thing you have and put in a hydrovac, you will have plenty of stopping power. I have several 50's models with hydrovacs and they have very good stopping power. I drove these type of vehicles when they were new. If you absolutely want a dual system, you need to get a system that is matched. Either take an entire system from a later vehicle, or get a complete aftermarket system that is engineered to work together. Don't try to mix and match or you will have either front or rear wheels grabbing which is very dangerous on wet pavement. The only big improvement I have seen in brakes on larger trucks in the last 50 years is ABS. That goes for air brakes also.