Thanks for all the info! I guess I should have been more specific. The TE20 block is a z120 block, not a Standard block. I am very familiar with timing a TE20 tractor, since I have one of those also. However, the tractor I am rebuilding is 100% To20 except for the replaced block and, therefore, does not have the timing marks on the pully and timing cover. It does have the regular To20 flywheel with the timing numbers cast into it. It just sounded sensible to bore the hole to see those numbers and add the necessary timing marks on the outside.
One thing I have learned (the hard way, as usual) is that the word "sensible" in dealing with tractors is sometimes the wrong thing to say!