1970 340 Duster rare x car.

Sounds like someone built the engine out of the DC catalog. Battleship spring, .557 or .590, and on and on.

You've had it 27 years, what about the prior 14? Lots of stuff happens and plenty of lore gets spun. Post some pics, especially the fender tag and broadcast sheet.

Never heard or seen a so called "X" duster. There are some "special" cars out there, never haveI seen even one that 's been referred to as a Duster 340X. I've been into these cars all my life with many people running stock and super stock from the 70's and on. Unless I saw something definitive, sounds like a puffing deal, like the 440 hp block is better than non hp ones, etc.

Post pics and an asking price.

Good luck with your sale.