FYI - I purchased 3/16" x 1" steel at the local hardware store, sandwiched it between the two maker slides and used 50mm “furniture hardware” bolts and nuts (flat heads). Works perfectly. The only downside is that I obstructed the passage through which the 2nd y axis stepper wire passes. That wire now runs around the back side of my machine to connect to the other y axis stepper.
I also used 1/4" aluminum plate to recreate the z-axis spindle carriage. I extended its height by 1" and added a third set of v-wheels. This allowed me to place the two universal spindle mount brackets farther apart. These two mods have virtually eliminated flex on Z and X. I can now run 1/4" flat end mill at .2" x 40"/min with no chatter and perfectly smooth cuts. I milled a circular pocket in poplar and the bottom was like glass. This would have been disastrous before the mods. I am using a 600watt spindle driven at only 48volts which yeilds only 6600 rpms (it is a 110 vdc spindle but my new power supply has not yet arrived from China).
Today, I am adding a single mid-beam support to each y-axis slide and I believe I will be good to go. I will try to post pictures if anyone is interested.