So if the tranny has a typical slip joint setup to the drive shaft is it safe to assume that it has 27 splines on the input shaft? I have a '66 dart w/ a 2-bbl 273 and my 904 doesn't have a ball and trunnion...
No. Early isn't defined as just the ball and trunnion units. Early is 62-67 and late is 68 and up. Your 66 will be a 18 spline input shaft and have the small 1.55" pilot hub.