Very cool project you are taking on... Fantastic rebuild so far and the machine will be sweet when your done i am sure. If you can pull off going to roller verses the ball rails and keep all the other alignments working, more power to you. Personally i would just stick with the same rails, not sure you will gain that much rigidity for the effort. I have 10 Fadals. I have a 1997 15xt. I do a lot of cast-iron work, and the 15xt i have will push a 2" spade drill thru the cast-iron at 300rpm and 2 inch a minute 7" deep thru the part and not using coolant thru with a G81 cycle. Big fan of the Wye-Delta 7500 drive on these machines. I still hope you can do the Rollers like i said cool idea. Another thing to consider in the future rebuilds especially if you are looking for rigidity is to go with the 15 and the15xt's Higher end Twins which are the 2216L and the 3016L everything about them is basically the same as the 15 and 15xt the main difference is the Column and Base are Cast iron verses the steel weldment Column and Base(the base is filled with Cement to dampen absorb noise and vibration ) while cutting.
Anyways Good Luck Looking Great and looking forward to the finish project. If your in the San Fernando valley look us up some of the original fadal guys work here come over and meet the "L" .