Have the 94 with 3vze apart to replace the HG's and would like to have the head surface shaved to the minimal amount just to clean then up and help insure a better seal. Curious as to what the the max limitation is.

 

Usually companies will sell head gaskets of various thicknesses to make up for shaved blocks/heads to retain stock compression ratio. I've seen up to 1.5mm over stock thickness, but I probably wouldn't want to go that much.

It doesn't take much to smooth out the head though. You can usually shave them several times before you need to start worrying.