Recommendations for driveshaft balancing in ABQ?

Additionally: How much extra space should there be on the end of the driveshaft that goes into the splines on the transmission (slip yoke)? We put a whole new 904 in this car, and I noticed that the driveshaft doesn't seem to go into the transmission as far as it did in the original transmission. The way I've judged this is looking at the exposed, shiny (non greasy) part of the slip yoke. In the new transmission, there's more of the slip yoke exposed outside the rear end of the transmission. But that may just be because the new transmission has a shorter rubber boot!! I wouldn't expect this 904 to be any shorter than the original one! (To my knowledge newer 904s weren't shorter than the earlier 904s.

But it's my understanding that the slip yoke shouldn't bottom out in the transmission anyways. Correct me if I'm wrong and need a longer driveshaft!