Wish i had joined earlier.
I have had a 5 1/4" 3 jaw on a vertex 8" rotary table with 4 slots for 11 years. Its done a lot of work.
http://pic20.picturetrail.com/VOL15/...2/14584660.jpg
Take chuck apart and you will find an area you can drill straight through. I used 6 M5 cap heads going in to self made T slot nut, 4 or 5 direct in to table tapped M5.
Did originally make a boss to centralise the chuck in to MT3 taper but would easilly go out of true.