Why am I getting a weird measurement for a driveshaft?

Your putting it in a Dart 111" wheel base is my guess. 904 and 8 3/4 is about 52.5" with 111" WB. Barracuda with 108" would be about 3 inches shorter at about 49.5"

These are theoretical numbers based on my research of what the books say. I have yet to mount my 8 3/4 and measure.

Thanks but my Dart is a Sport so it's the same as a Duster... 108" wheelbase.