Yes the parts should interchange. I just swapped the entire rear half of a 55 in place of the original one on my 550, due to someones attempt at repairing a transmission gear involved welding bearing retainers and such. It was easier to just swap &quot;transmissions&quot; then to attempt the rebuild.

There are some differences between PTO assemblies, but they are minor. The potential number of discs and a greaseable bearing. You could easily swap one for the other. 

The case that the PTO and engine clutch are different.  The 550 has holes for the grease hoses/fittings to the throw-out bearing and for the PTO clutch. The 55 will bolt in the same as the 550. 

Oliver didn't seem to make any major changes between the models, just minor improvements. I don't think you'll have any trouble with interchanging.