Ted,
When I did my 54, way back years ago I used a disc brake conversion from www.rustyhope.com. It worked pretty well, comes with the caliper brackets, the seat slip on ring, and a list of parts that make it work so you can source your own parts.

There are probably a lot of more modern kits out there, probably some that use current off the shelf parts.

The last several trucks I've just clipped with a Dakota frame, but that is a lot of extra work if all you need it a disc brake set up. Gene