While the tailshaft of a later tranny won't work with the earlier mainshaft, you CAN use most of the parts interchangeably. You should be able to use the good internal parts from the '70 tranny to make the '73 tranny serviceable. It really isn't very hard to disassemble an Opel transmission, so long as you have a factory service manual and CAREFULLY follow the steps. The pictures are quite easy to see how it comes apart and goes back together, and the text is very understandable.
You need a few basic tools: a bench vice, a set of circlip pliers, a brass drift, and a 1/8 inch (decent quality) drift to remove the roll pins. To remove the internal synchro rings (2nd and 3rd gear) you need to press the gear clusters off the main shaft, but any machine shop should have a bearing press that can accomplish that. The manual will call for a few special tools, but a bit of common sense can get around them. For example, to remove the counter gear cluster, it calls for a special tool to drift out the internal shaft while retaining the internal needle bearings. But a brass drift of approximately the right length and diameter will do, and the needles can be re-loaded (24 per side) by packing them in grease. The detent spring plugs are supposed to be pulled with a special tool and slide hammer, but a blunt chisel and a few upwards-directed hits will pop them out.
If the '73 tranny won't shift properly anyway, try disassembling it to see what has gone wrong. Look for worn synchro rings; a worn ring will not leave an gap between it and the matching gear, if the cone has worn. It is hard to explain, but it will be easy to see what I mean. And look for obviously worn gear teeth and damaged bearings. Virtually all of the major bearings and seals are available from either a quality bearing and seal supply store, or from OGTS.
HTH, and good luck