Dakota,
Here is what I did when I modified my Lee mould. First of all, no milling machine work is involved, you just need access to a lathe, a drill press and a set of drills and taps.
1. Remove the sprue plate. This is held in place with a self tapping screw and can be a b***h to remove, but it does come out.
2. Carefully locate on the top of the mould blocks the centers of the pins that hold the handles to the mould and center punch these locations.
3. Drill down through these located centers with a 7/64" drill untill you hit the hole that the pin is in and then drive out the handle holding pins with a 3/32" dia pin punch.
4. In the lathe, in a four jaw check, modify the mould by cutting off as much from the top of the mould as desired.
5. Using the existing sprue cutter screw hole, deepen this hole using a No. 21 drill and deepen the tapped hole using a 10-32 tap.
6. Using the holes for the original handle mounting pins, drill from the bottom, using a No. 29 drill, all the the way through the blocks until the drill exits the top of the mould blocks.
7. Tap these two holes, from the bottom, using an 8-32 tap all the way through the mould blocks.
8. Open up the holes in the handles to allow the free passage of an 8-32 screw without binding.
9. I then use the same screws to secure the handles as the ones that are are furnished with a Lyman mould and this now makes the handles removable.
Hope that this helps.
Hell-Er High Water