What size allen head bolts secures the driveshaft for SPF?

[Pman1961]

My bolts securing the driveshaft to the differential are the standard 6 point which requires an allen head. I have a 3/8" which is to small and I believe a 1/2" which is to large. I did not know if this was a 7/16" or metric. My plan is to purchase an allen head which would work on a ratchet. As far as installing the engine and transmission as a unit or seperate I had a thread started some time ago about the best method for removal and installation. I had an equal response on going in as a unit or seperate. If I had a chain hoist from the ceiling I would consider installing as a unit. I do have a tilter. My engine hoist is not long enough to do a front approach. I pulled the engine out seperate and with the tranny tunnel still in place I had to rotate the tranny 90 degrees to get it out of the shifter hole. I'm going to have a friend come over to help me lower the engine in place and then I was going to try to come up with some kind of cradle to hoist the tranny in place or near enough to where good ole muscle does the job. Of course with this method I need to remove the driveshaft. I was originally going to set the tranny in first then lowering the engine and join the two together but I was afraid off not being able to feel the two mating together properly or possibly bending the clutch diaphram or damaging the pilot bearing. Would this be worth a try or is there not enough room in the engine compartment to mate the two together in the lower position?