First: Road Runner…never “Roadrunner”!
If it is a real Road Runner, the VIN on the dash, and data plate will start with “RM”, and the last 6-7 digits will be stamped on the radiator yoke. As you guessed, the standard engine was a 383 — a big-block, making me suspect that it isn’t a real RR at all.
As you mentioned, In Violet paint (Plum Crazy on a Dodge) was not available in 1969; 1970 was the first year.
A big-block or Hemi must be mated to an A-727-A transmission, there’s no such thing as an A-904 that will bolt to either engine. To be done correctly, a whole slew of things need to change, esp. to a Hemi.
As far as what to do with it, check the VIN. If it is, amazingly, a real RR, maximum value is typically obtained by restoring it as close as possible to original. If it is a rebadged Satellite, you may as well do as you wish with it — maybe a 3G Hemi? (5.7L / 6.4L etc.)