Hub caps for 13" wheels

Hi Wally,
What I've found is that among 13" wheels they're not created the same. My 64 dart /6 lost a rim to a blowout so I replaced it with a junkyard donor. This slightly different wheel (still a 60's Mopar) doesn't hold the wheel covers quite right and they will spin slightly under braking and **** the valve stem to the side.
I read somewhere that the 9" drum and 10" drum wheels are different.
Maybe your wheels and wheel covers are mismatched?