No custom milling needed if you just provide the right mounting holes on the board and position the chips correctly under the heatsink.
It is entirely possible to create 1.0v power supplies that deliver up to 120 amps with very little difficulty using polyphase buck converters. For instance, a 6 phase system need only handle 20 amps per mosfet/inductor, but once you have gone that far, it doesn't cost that much more in parts to have an independent 12-15 amp power supply for each chip.