Depending on the type of machine you have you may be able to jog your gantry past the edge of the bed and clamp the rails to the front of the machine. My gantry goes long ways along the y axis so i can have the spindle extend past the front of the machine about 4 inches.
Another way may be to bolt on a angle plate. then bolt your spindle bracket to that. In the attached pic I used a piece of 8"x8" 1/2" wall tube and bolted it to my Z. Then I can mount my spindle vertical or horizontal. I paid $50 for the tube. You can get a 8x6x5 webbed angle plate from enco for $20. Ductil iron is much better than steel. It won't transmit vibrations like steel.
Hope this helps.
Donny |