Interchange question

66 to 76 valiant, dart, and up to 69(IIRC) barracuda are all interchangeable.
If it was originally a slant 6, 273 2bbl ,318 2bbl it would be a 7 1/4, 340, 383 would be 8 3/4, 440 it would be dana 60. Later years some time in the early 70s had a 8 1/4,

The cars fender tag will have a code that will tell you what it originally came with