If you’re using a stock type transmission mount bushing that has a male stud hanging out of it, you can not use this type of mount with some of our crossmembers. We recommend using a polyurethane transmission mount, or if you want to stay with rubber, get a transmission mount from a 1965 V-8 Chevelle (available at any local auto parts store), this mount has two female holes just like a polyurethane mount does.
If using a one bolt tranny mount place it through the center hole in the crossmember. If using a polyurethane mount, or an early GM type mount as mentioned above, use the outer two holes in the crossmember using the two 7/16" bolts that were supplied with your crossmember.