Okay, just machined the mount (1.566 in. wide, 1.231 in. high and 1.066 inch thick). With the 0.875 in. dia. hole for the 15/16-16 tap, I end up with about 0.290 in. on both sides of the tapped hole.
So, that basically eliminates the use of 1/4-20 for mounting the mount to the underside of the y-axis table, 10-32 requires about 0.190 max., so that moves me into the 8-32 with a 0.164 max.
What do you think of using 8-32 SHCS that are going to be about 1 1/4 to 1 1/2 inch long and go into the mount about 0.6 inches? That would be how the ballnut mount is attached to the y-axis plate. :devious: