Hi Dave
I made my 4" raising block for the VMC in cast iron purchased from M-Machine. A bit more expensive, but machines nicely.
Mine was made in 2 parts as Gary has suggested, and was written up for MEW, starting in issue 215. I understand that the position of the hold-down studs can vary, presumably depending on age and sourcing by Warco, so it might be worth checking your machine before you make the raising block.
One little problem I have experienced when testing the Z-range is that it is possible to disengage the Z-axis leadscrew. Had great fun re-engaging it and will not repeat the exercise.