The Shimano 600 headset has one of the lowest stack height spec's of any headset and if you're doing a direct exchange, your options will be very limited. If your fork is already cut anfd there's no spacer in there, I'd suggest you find a Dura Ace, DA AX, DA Track, 600 EX, 600 or 105 headset as the best replacement.

On the vO 40mm stack, 2mm is pretty small pickins and might just work.