I think THE FILLING STATION has radiator caps, I have a 29 coach ,a 30 truck like yours and my son and I are build an old time doodlebug out of a 1930 parts car, I did have some extra 28-30 radiator caps , not sure if I still have them.
Yes the Model A Ford did not use a same cap as the 28-30 Chevy did .27 Chevy has a smaller cap and 31 has the twist lock [not threaded] type of cap, I'll look around and see if I've got an extra , but I beleive they are available. Iknow I've got one with a hole in it.