I have a emco mill 50 that I plan to convert to mach at some point.

I would start with mounting your new stepper motors. I think the oringinal pulleys are actually held in place using a roll pin straight through the pulley and shaft, but I have seen people just using grub screws or even glue. Get the old steppers out and have a look, spend some time just looking over the machine to formulate a plan and post plenty of pictures here to get advice and fill us all in.

Have you been using the mill 55 with the old software? Alot of folks say there is no need to convert to mach as the software is actually not to bad.