Not a very good idea to convert a race bike into a crossbike, the frame's geometry is very different, crossbikes are longer , have a higher bracket ,slacker angles and longer chainstays.
This makes them much more stable riding through dirt , uphill and downhill.
I personally would transform the Mtb into something racy and light , with slimmer tyres , say 26x1.4" at the most and try it like this.
You have to adapt your chainrings too, 34 and 46 in the front and 12 to 25 8-speed in the rear.This will make your riding faster, harder and less complicated.
You will fall less often than riding the racebike on dirt and have a great time !