Hi, I'm converting the same machine and using the same motors (running bipolar parallel at 4.2A) and drivers as you are. The only exception is that I have milled nema mounts manually.

You're right about the collar, but there's little chance of anything going wrong with both collar and coupler locked up nice and tight. I'm only using one locking screw on my Z and it's fine.

I noticed your other post on your driver/motor issues and I'd be glad to post my settings and steps,timings and dip switch postions if you still need a rough guide to get them running better. I just completed all 3 axis last night and they're running fairly smoothly, although this machine needs constant tweaking to keep it running sweet, or so I'm told!