How do I find an 8 3/4 rear in the jy

If you want bolt on.

Your looking for:
66 thru 72 Dart first digit is a L
70? Thru 72 duster, demon etc.
66 thru 69 Barracuda first digit is a B
66 thru ??? Valient. First digit is a V (IIRC)

Look for VINs that have I higher letter in the 5th spot than a D


Looking under the hood look for V8s with a 4bbl

Trailer hitch in the back is also a possibility

66 has the vin on the door piller and is silver in color
67 is on the door piller and is black in color
68 up is on the dash