Ron,
I believe your bases have already been "skim"milled to level them(when orig. installed).It would be very difficult to scrape them in level, and milling them level is easy(they have to be close though, or milling removes too much).The bases are not often the same height, just as you thought;this is where the boresighting/measuring distance to base comes in.The chord isn't used to tie the scope to the adjustable block,rather it is to fasten the scope to the rifle, with the block trapped between( you need to be able to remove the block to adjust it).
The depth gage at the end of the caliper(or any other type)is used to measure from the top of the base to the top of the barrels,through the opening for the foot.Measure to the lowest spot,mill the foot to this length, then contour it to approximately fit the barrels.The foot should't bear on the barrels, though, or it may interfere with other fitting.
Once the foot is fitup(the slider and springs should have been removed for this operation),you can reach in through the slots on the side and scribe a line on each side with a sharp scriber.If you don't have one, it can be easily made.
If you can find a gun with clawmounts that have been mounted in Germany, or by a Germany trained gunsmith, inspecting it closely can help you a lot, to see how deep to make the slot in the foot for the slider,the angles, etc.The trouble with learning through trial and error is that the cost of the error can be pretty high, if you have to buy new parts.Let us know how it works out, and as always, I'll do what I can to help.
Mike
