I have a call in to the possible 1st owner in Texas and he may be able to enlighten us on it's history. Stay tuned.
I am sure a few of these were blown up early in the warranty too... That would be a good case for an early replacement block.
Thank you for the compliment...Make my next project a Charger please! Nice.

Dirk, the reason I say it's probably not a warranty engine is that the "D" stamped on the upper pad would not be there indicating the model year it was originally installed into. I am with Dan on the HP vs non HP theory, as well as the possible date correct replacement, ESPECIALLY with the lack of an "HP" on the pad after the D 440.

Would that then indicate someone was conscious enough to replace the original motor for whatever reason with a nice early block out of a non-performance Mopar?
Would a GTX only have an HP block?

Yes, either on purpose or by mistake someone used a "heart-donor" early non-HP '68 car.

all '68 GTXs were 440HP (375hp) or HEMI (as were Charger R/Ts & Coronet R/Ts)

in '68 some c-bodies were available with 440 (350hp) or 440HP (375hp)

FYI, all '68 440s were painted turquoise.

What's this number stamped on the passenger side bellhousing about the fill tube?