John is right, but ever since the 60s many have put larger and wider wheels on the car seeking to put more rubber on the road. Search this forum about tire size and you will find differing opinions (beyond what was original). I had 15", wider on the rear, when I got my 68, but I just switched back to 14" BFG Radial T/As and the car rides better and doesn't scrape.
Kurt is right. Why don't you share a bit about your automotive experience; specifically describe problems or issues, and ask specific questions? That will greatly add understanding for the experts here at CRG in giving you good advice. I've learned and learned and learned from these guys!
And hey, post us a picture of this 68 convertible! In my
opinion we can never see enough 68s...