Having read your earlier post and having many years experience of the Multifix tool system. I would not entertain trying to modify the toolholders. From memory these are case hardened steel. Removal of 3 mm of material from the tool slot will leave a soft clamping area for the tooling.
Removal of this amount of material from the base of each toolholder, such as to retain the versatility of the system does not come without problems.
You may find the removal of this material distorts the ground mounting face which may affect the repeatability. For my part I would sell this item and obtain something more suitable. Or use smaller tooling.
When I had my own 254, Myford supplied it with a fitted Dickson system.
Regards
Gray,