19.5 rims are popular with "LoPro" medium trucks, but I see the truck you pictured has the old Dayton style hubs and rims.

You would have to switch your axles out to hub piloted Budds from a later truck. That would get you better (Disc) brakes, although all of the recent trucks have antilock systems which is a further complication.

Unless you can pick up a decommissioned school bus donor cheap, you may want to just enjoy the old girl for what she is: a 50 mph local hauler.