I have just finished the Hoss conversion and so far it's working very well.
Like others I didn't have a boring head to make the 2.338 inch hole in the z-axis ballscrew mount. I solved this by finishing the x and Y axes first and then using the spiral pocket wizard in Mach 3 to make the hole, adjusting the z axis manually as I repeated the g-code. I also used it for the 1.255 through hole. It worked perfectly and there was no wasted tear-down, re-build effort since the z-axis is totally separate from the other two in regards to that.
By the way here are some observations/comments on my conversion:
1. I used Roton ballscrews. The square ballnut profile interfered greatly with the X axis cavity. I ground the ballnut down as far as I comfortably could and then had to grind the underneath cavity of the X axis quite a bit.
2. I reused the X and Y axis handwheels.
3. I ground the threads off of the ballscrews before machining them in my mini-lathe.
4. I used Lovejoy flexible shaft couplings from Enco. These are very nice but the X axis stepper spacers (I use 1/2" round instead of hex) need to be slight longer than specified for them to fit.
5. The Z axis took a bit of fitting to eliminate binding. The placement of the base block (i.e. the bottom of the ballscrew) is critical. Generally it seems that if it doesn't bind near the top of the Z axis travel it is fine as it goes lower.