A slant six would have a 904 behind it.
If you want to restore it to stock, parts may be hard to find, but based on the photos in your other threads, the trim pieces and other things are in good shape. So long as the metal is in as good a shape as it looks in the photos, you won't need any new sheetmetal, which is a good thing, since no one makes it for the 1stgen Barracudas. I also doubt you'd be able to find a new dashpad, but you can send it off to Just Dashes and they can restore it.
The good thing is that there are plenty of pieces from other Mopar iron that are fairly easy to swap into it, so if you and your dad are looking at upgrading the car, you should be able to do so without too much trouble.