Cruise control will not work .

Since a 68 never came with cruise, what are you using for the cruise unit - electronic aftermarket or OEM vacuum/cable adapted to your engine? Pics would help too!
ok.
it's a oem vac/cable operated unit to a 1968 340 gts 4v auto . I believe dealer installed. I get pics up later .
It makes sense that if the speedo needle in dash is jumping around the cruise unit can't focus in on a set steady mile per speed.

1968-dodge-dart-gts-convertible-3.jpg