I will be interested to see how you go.
I was looking to do a similar thing a while ago but have not gone through with it.
Too many other things going on.

I was heading in fitting 'extrusion for the gantry' route. Maybe 135 x 45? I was looking at a Linear Rail conversion.( 45 series allows easy bearing block mounting)
It seemed alot of work and then your weakness will be your Supported Rails for the Y axis, but that may not be a deal breaker....