[spiderserie4]

the normal bolts go through a hole that is 9.144 mm (0.360")
the shoulder bolt goes through a hole that is 10.16mm (0.40")

not much help I know, but at least you know bigger than that and it wont go thru

...Tifosi had a good drawing of the shim/spacer that you can see in the above photo, from which I got these measurements!

Remember the shoulder bolt goes in the other way around to the remaining two normal bolts (again, as you can also see in the photo above)

 

[spiderserie4]

can't remember anything special about the engine mount/support rod thru bolt, except that it has a smooth shaft (it has to go through the support rod with no play) and it is quite a tight fit. The nut faces the front of the car.
If I remember correctly the nut wrench size was one up from the motor mount nuts themselves, so 14mm I think (or maybe 15mm)
It is part 2105.02024 on this diagram