On my '66, the right motor-mount bracket fastens to two slotted holes in the frame, and one circular hole. The slotted holes are at the top. The left-side bracket fastens to all circular holes. 3/8" bolts all around, but the bolts for the slotted holes are specified with flat washers.
So what's the purpose of the two slotted holes? Were it not for the circular fastening hole, they'd allow longitudinal movement.

 

The two slotted holes would allow for a small amount of twist, pivoting on the circular hole.

I don't know why that would be needed. I'd expect the rubber mount would have plenty of compliance.