Differences in 318 block.

3.931, less 3.910 =.021, which is a chitload of wear on a "std" bore.
Was this measured with a dial-bore gauge? If yes, then,
more likely is that the engine is already overbored.
But if no, not by dial-bore gauge, then, no comment.

so to answer your question, as to .020; it would kindof depend on if the bores are straight and/or how badly tapered they are. So then, this needs to be measured with a dial-bore gauge, to confirm.