Neat truck ya got there, not something ya see every day!
1st & easiest thing you can do is check the adjustment on the brake shoes. Less gap between the shoe and the drum = less fluid needed to push them. Sometimes adjusting the shoes and a quick bleed can make a lot of difference.
I have a '56 2-ton dump truck, after getting everything back to factory standards it will stop very quickly with a full load. Making sure everything is adjusted and the booster is functioning properly makes a lot of difference.
Disc conversion is a possibility, but the cost & availability of parts,... It's not as easy as it sounds, someone else here has more info on your year model I'm sure.
However, a more immediate help could be converting from HydroVac, asuming yours is even working properly, to a Hydro-Booster. There's a bit of info on here about that trick as well.
