If your '55 transmission has a 29 spline input shaft, then all you need to do is to take out the 4 bolts holding the front pump and swap pumps. If it has the more likely 17 spline input, then your converter won't fit the FMX unless you either swap input shafts (complete teardown of both trans) or get a converter with the 29 spline turbine. If you are having yours rebuilt, maybe the rebuilder can put the correct spline in the converter for you.
If you chose to adapt the FMX converter, you will need a flywheel from a '59 or later Y block. The Bird converter has the starter ring gear on it, the FMX does not. The later flywheel does. I guess a converter rebuilder could put a ring gear on an FMX converter, have never looked into that.
John - "The Hoosier Hurricane"
