Sounds like a cool project.
Millions of cars had single master cylinders and they lasted decades without real problems as yours did but it's one of those things you'll always be wondering about. Whether you change to a dual or rebuild what you have, the key is not to have any weak links by making sure you address every single item in the brakes to make sure they are 100%. The lines especially with the age as you found out before. If anything is marginal, you can lose all brakes with a dual as you can with a single. Been there. Don't forget to get the parking brake up to 100% too and use it so it works in an emergency.
I'm sure someone will jump in that has done the same truck and ironed out the wrinkles already.
