I've got rally wheels on mine, center caps pop right in with the smack of a fist to the center. Do you have the regular derby centers, or something else. Also, where did you get the wheels and caps? Mine were oem GM, maybe reproduction ones don't fit as well, or maybe when new they take a little more agressive pounding to install.
The center caps are the '67 style reproductions from Wheel Vintiques. It almost seems like they're too large, but I didn't think rally wheels were any larger than 15" so I don't understand what size wheel they could be for if not for the 15". Besides, they were bought specifically for 15" rallys. I tried using a rubber mallet and it started to dent the inner lip of the cap on one of the three protruding knobbies of the rally wheel.
Is it better to try fitting the cap past two of the three knobbies and hammer away to get it past the third or to center the cap equally on each and then try?