The build sheet shows: 350 V8 2BBL; 4 Spd; PB Radio, Front Manual Antenna; Rally II Wheels; Custom Sports Steering Wheel; Console; Disc Brakes, Front; Soft Ray Glass, All Windows; Custom Pedal Trim Plates; Power Top; 195R x 14 White Radial Tires; 3:23 Axle. Everything matches the car except for the rear antenna and the additions of AC, Tilt Wheel, and Dual Exhaust.
Based on this, I think not having the original data tag is a slight knock, but not a big thing. I'd say the stock 'upgrades' increase the value of the car much more than the slight decrease due to the missing tag. I think technically it is a (legal) crime to remove or replace the tag in the US. So personally, I would not mess that up more than it already was by a previous Owner. Good luck with the sale, that is a very nice 'bird!
I used to be indecisive. Now I'm not sure. I feel like I am diagonally parked in a parallel universe. 1968 400 convertible (Scarlet) 1976 T/A - 455 LE (No Burt) 1976 T/A New baby, starting full restoration. 1968 350 - 4 speed 'vert - 400 clone (the Beast!) 1968 350 convertible - Wife's car now- 400 clone (Aleutian Blue) (Blue Angel) 2008 Durango - DD 2008 GXP - New one from NH is AWESOME! 2017 Durango Citadel - Modern is nice! HEMI is amazing! 1998 Silverado Z71 - Father-daughter project 1968 400 coupe - R/A clone (Blue Pearl) (sold) 1967 326 convertible - Sold 1980 T/A SE Bandit - Sold