Will front disc brakes off a 69 dart fit a 74 scamp???

It's a common conversion to use LBP spindles and disc brakes from a '73-up A-body on '69 Darts. The very same conversion was done to my '69 Dart before I bought it.
The LBP brakes on the '69 Dart you're looking at will fit perfectly on your '74 Scamp. Just make sure to verify they're large ball joint spindles as said by crackedback.
Then buy aftermarket LBP rear axles for your 8-3/4 and you'll have matching bolt patterns. You'll also have a MUCH larger selection of wheels to choose from.