There were a couple of discussions, both here and on the H.A.M.B. about he proper length plugs for Offenhauser heads. The general conclusion was that they don't seem to be designed for any particular length plug, and that for optimum use, spacers need to be used. Here's a link to one thread on the H.A.M.B. :
https://www.jalopyjournal.com/forum/...#post-13317505. (It's interesting that the H.A.M.B. thread ends up pointing back here to "The Barn".)
Note that it ends with "flatjack" (who really knows his stuff) saying that the equivalent of a Champion "N" series needs to be used
with a .21 spacer.
In any event, stock 7/16" reach plugs are not correct for use with Offenhauser (and most other aftermarket) heads.