That's the one dimension that is all fecked up on the internet.
Some have tried to create charts only to find errors in the info their research provided.
Ways to go...
1. Find someone with the exact same combination of wheel base model, trans, and rear ( good luck ).
2. Tape measure
Because you are working with a non factory set up for that year as was mentioned the best way is to measure. If you have a 904 trans yoke slide it in the trans (at proper depth) and measure from the center of the trans yoke U-Joint to the center of the rear carrier yoke u-joint and post that measurement info. I have all kinds of tube diameter & lengths of drive shafts available and may have what you need