66 Plymouth Valiant help!

48 years and nobody died in it yet.
You can upgrade to a split brake system without changing anything else.
You could replace everything from the radiator cap down.
It boils down to how much money you want to spend on a 66 Valiant.
Just keep in mind... The more you change from OEM the more difficult it will be for the next owner to figure out what replacement parts they need.