Jim Carter offers 6 lug disc brakes for your truck. You could also get them from Buffalo.
When I put the dual master with booster on my 59, I had to relocate a pulley for the ebrake back a few inches on the frame. Everything else was straight forward, The new master will be way under the seat. Typically you check fluid in it by using your finger. You can use a squeeze bottle to fill it up.
