P225/70R15 is the proper size for your car. 60-series tires are favored by many but they may or may not cause clearance problems...it seems to be a car by car thing. 60-series tires weren't authorized by GM until the '78 model year after some internal body panel changes were instituted.
As far as what brand, that's a fast way to start a fight. There's three ways to start a flame war...ask about the best tires, motor oil or wax for a car. That's a can of worms that steps on a lot of toes.
Whether you find black wall tires, solid white letter, outline white letter or white or red stripe tires is a very personal decision. Decide what you want and then look for the tires that best suit your decision. Availability makes a difference as well...the size tires a C3 uses are more or less obsolete today and while they are available, manufacturers generally only make occasional runs of them...they're not in full time production, so some are not always available when you look for them.
Not to step on anyone's toes here, I will say from personal experience, that I can recommend the BFG Radial T/A, which seems to be the standard which everyone compares they're tires to. The T/A simply looks good on a car. I can also recommend the Cooper Cobra...an excellent tire and the name "Cobra" on it can have a side benefit as it irks the Ford guys. The Pep Boys Futura GLS is nothing but a Cooper Cobra with their name on it as Cooper makes it for Pep Boys. The Dunlop GT Qualifier is another god one, but I'm not sure if it's still being made. Many have said very good things about the Firestone Firehawk...I have no personal experience with them, but I have read many good things about them.
Decide what best suits your purposes and needs and start looking and comparing. You have lots of choices.
