Numbers Matching?

The '68 casting date 340 in my Duster is .060" over and is putting out north of 400hp without any problems. Not saying every single one that left the factory will go .060", but it wasn't an issue on mine. And I would suspect it wouldn't be an issue on the majority of them.

As far as the number matching thing, unless that '73 340 is your long lost engine, it won't add much if anything to the value. On some of the high dollar auction cars having a date code correct, non-numbers engine might add enough to be worthwhile, but on a '73 Duster it won't be enough to bother.