With the arm straight a cross from the transmission, measure from the inside of the cup, to the deepest point, to the other. That’s how long that one axle needs to be. Measure the other side the same way in case to car was built a little off.
Once you get measurement from inside of one flange to the other you need to subtract anywhere from a quarter to a half an inch. If you buy an axle with this much play it will allow for the perfect amount of plunge and won't bind or grind. Make sure when you measure your axles that they are perfectly straight across from one another.
Another way is to find a junk axle and cut it in half. Make a sleeve with pinch bolts or hose clamps and build an adjustable axle. You can cycle the suspension and check it for proper length at full droop and through the travel. About 3/8" play at full droop is safe.
You must have adjustable type rear trailing arms. I would get your rear end as aligned as possible and then measure from inside of one flange to the other. Or like the other guy said with cutting used axles and sleving them. This would make them adjustable to make sure they cycle ok without binding you cv's. You should be safe though if you have a quarter to a half of inch of play when your car is aligned.