I don't see a real reason for using carriage bolts.
I haven't got to this yet, so not replying with any real knowledge ...
But my understanding was the carriage bolts are so they don't rotate in the timing plate when tightening them down (with std bolt heads in 'normal' orientation, otherwise difficult to acces)
But inverted, with a lock washer (or maybe even a star washer) under the nut, would imagine likely tighten from the bolt inserted from underside even without a wrench on the nut.
However I'm not really sure why those bolts are advised to be 'loosely' assembled until after it is through the crossmember? I guess because final torque would be with the load of the motor on the mount?
It's not like alignment of those can change, especially if they were carriage bolts
Apologies for diversion @Poncho52 - hopefully my first reply got you what you needed.
