You might be able to back out the trans to block bolts till the last couple of threads remain, undo the trans mounts bolts, and carefully ease back the trans enough to get those dowels in position each side and then push trans back in place?
Or he can remove 3 bolts and install longer bolts in those 3 spots then remove all the other bolts. That should give him plenty of slack to back the transmission up enough to install the pins.
