Just out of curiosity and the fact that I have been thinking of using the later steelies on the 48 with small hubcaps I sent an email to
http://hotrodhubcaps.com/chevy_hubcaps.html and Jeremy answered back today that their caps fit the later style wheels.
I'd measure the diameter of the circle of the bumps that the cap snapped over and send off an email to see if their caps will fit your wheels.
Their price isn't cheap but it beats the expense and trouble of getting another set of wheels (that may not run tubeless) Painting them and the expense of having the tires swapped over.