1968 Dodge Dart Driveshaft
If you have a drive haft yoke, slide it in until out bottoms out, pull out about 3/4"-1" an measure yoke to yoke on center best you can. If your working with a drive shaft shop ask them.
Was an 8-3/4 ever offered with a 904? Seems like if someone ordered a heavy duty rear end they would have got a 727.