Checking Sprag

I just got my new Dart bracket car ready for it's first test & tune.
Wanted to make a couple of shake down passes off the footbrake before going to the transbrake.
At the first hit,it broke the rear u-joint instantly,it was a 1350 to stock Dana conversion joint.
Is there any way to check the sprag with the tranny in the car?
I spun the tailshaft with a spare input shaft and could not hear or feel anything out of normal.
Thanks