727 guru question...

If you search youtube for John Cope Racing you will find he has a bunch of short tech videos posted. One of them discussed the differences and showed examples of the differences to watch out for. Basically, consider the main case, the front section, to be the universal part. Get the one the fits your engine (big block). Then always keep the output shaft matched to the tail housing due length and installation type, such as slip yolk, drum parking brake, or the output flange secured with a nut. The fun starts with keeping the gear set splines matched to the shaft splines, which is where the year differences really get important. Then, make sure the pump stator support matches the high/reverse (front) drum machining for seal rings and bushings.

The pictures in post 17 are all considered short shafts but it shows a bunch of the variations. Your 74 model year truck is getting really close to one of the spline angle changes, so be careful and keep your output shaft and gear sets together.

NOTE: Don't use this post as 100% accurate info. Watch the videos and look at the parts to be sure I did not mix up the details.