Hi
I have a 55 Ford Wagon and I have converted to the Disc Brake from a Granada it is a simple and easy swap and fairly easy to find in the bone yard. Two things you will need to do......
1) Ream out the ball joint holes on the spindle it doesn't take much.
2) Being the Granada brake lines enter the caliper from the Front of the car you may have to reroute the brake lines so they don't pinch when turned.
Other than that it takes about 3 hrs to convert it over to disc brakes
Oh almost forgot there is a differance between left and right on the calipers the brake lines will be on top facing towards the front of the car that is how you can tell and be sure bleeder valve is on top
Good luck with you Mercury
55 Ford 2dr