Are you Wolfie still intending to use it as a drilling only vice? If though you expect to use it for milling then look at MEW issue 23 which includes adding a longer moving jaw.
I agree with JasonU above. Fix a block of metal to the machine table and clamp the vice, using its own clamping jaws, to the block. You should attempt to remove the minimum so check that the base is reasonably parallel with the machine table.
Across the vice is easy, just slightly loosen the vice and tap the higher side down with a soft hammer. If out along the length you will have to place a shim under one edge of the block.
Don’t expect perfection, but aim to get within 0.1mm before skimming the base.
I will answer your PM’s later this evening, sorry for the delay
Harold