If you're using the same VB I will assume that you're using the same park rod that worked in other transmissions, the 904 and 727 park rods are different

With the VB removed, look into the hole that the park rod passes through and note the position of the snap ring that holds the tapered ramp into the tailhousing; in rare cases the position of the snap ring ears will interfere with the park rod operation. If the bottom ear is installed low in the housing the park rod can get wedged against the pawl.