Did the engine have shims originally! Many of those engine use shims to "dial in" your clearances. I've even seen service bulletins say that if your oil pressure falls off below a certain point, that an "adjustment" to the shims by your mechanic can bring back lost oil pressure. So I think that shims are an acceptable method for "tweaking" your clearances. Its just a little more time consuming. Most times, before a crank is turned, you can torque the bearings in the block and caps without shims, and have the crank turned to fit it,eliminating the shims altogether.
