I'm about 80% into an x2 conversion using these plans. To answer your backlash question - I went off on different track and tried this.
Making Acetal leadscrew nuts the easy way
So far they have been working very well.
I'm not using the airspring system, so I can't comment on that although I am planning on using a counter weight system rather than springs.
You can see some pics of progress here.
Mill Photos by madluther | Photobucket
I'm using 350 oz in servomotors on my conversion.
Regards.
Steve.