You're OK, the arms should have a rubber grommet that fits inside the hole. This rubber grommet will then work fine with the hex shape on the motor. Mine were hard as a rock. I'm sure all the vendors that specialize in the old trucks know what you need and have the part. Or maybe a local electronics store may have such small grommets, used for wiring passing through metal.
Once you replace these grommets then you secure the whole mess with this spring affair that clips over the 'hex' heads on the wiper motor. Clear as mud I'm sure, get a parts book or catalog from Carters or Chevs of 40s.

It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed