Brothers Trucks has a set but I'm not sure if these are the kind that will fit over the center of the rim or in the center. From the picture they look like they fit over. I don't know if they'll fit on the original wheels either.
http://www.brotherstrucks.com/prodinfo.asp?number=1009
This is exactly what I wanted to do on my '68 but could never find any baby moons. I wanted to put some baby moons on it like what came on the early 70's Camaro's with the bowtie stamped in the middle. I still have the original wheels from my '68 and if I can find some baby moons to fit them then I will go with that wheel set up. My uncle actually has a set of baby moons like those that came on the early 70's Camaro's but I can't get him to sell them to me, his dad passed them down to him and he has them on his 70 LWB now. Here's a pic of a '70 Camaro my dad bought brand new and it had those baby moons like I want. The '68 in the background is mine, but it was my grandfather's and the pic was taken a few years after my grandfather bought it new.