The important distance is the thread depth, which appears to be the same for both plugs. Extended-tip plugs put the tip of the insulated electrode out into the incoming fuel mixture, which tends to make the tip run cooler. The Delco 46 series is one step "hotter" than the original-equipment plug, (original was a 45,IMHO) so that should keep any deposits burned off the extended tip. I don't see a problem with running the Delco plug.
OOPS! I misread which plug you have now- - - -the short Autolite, right? If it's the equivalent of a Delco 45, you're still OK. The short plug with the equivalent heat range to a Delco 46 will be good for short-trip driving. It might tend to cause a little detonation on a long haul under heavy load.
Jerry
