84 C20 brakes

[TazmanB]

I bought an 84 GMC C20 single cab long bed that my 13 yoa son has been wanting for over a year. I got a really good deal on it and there is no rust/rot on it anywhere. The title shows it is a C10 and the vin matches the cab. So it looks like it is a C10 cab swap on to a C20 frame. It has disc/drum brakes with a master bolted to the firewall. There is a booster in the floor of the cab and the brake pedal goes to the floor, absolutely no brakes.

I am going to replace the entire brake system minus the hard lines. My question is as long as I replace everything and go with the JB6 or JB7 brakes does it matter what is on it now?

If it does matter what brakes are on it now how can I tell if it has the JB6 or the JB7 brakes on it now?