Opinions on poss. purchase of a 340 block

A magazine trick showed to take some fixed metal rods, and cut them and dress them to the sizing of a standard bore 340, and then 340 + .030 and so forth. Use a micrometer to make sure the length is precisely .030 over, and then run them in the bore to see if the bore is the side he says. If they are loose, then it is not .030 over.