[pfilean]

63 or 61 - may not be much difference. My 61 had 15x7 with about 4 5/16 backspace and 215/70R15 tires. I thought it had the tire to close to the outside of the fender and wanted to get more space. Couldn't seem to find a wheel listing that got a little more backspace even though I thought I could tolerate maybe and inch more backspace than the 15x7s. I have an additional problem in that if I did find a wheel with more backspace I would have a problem with my front tie rod ends. (I don't have stock Pontiac steering arms so it makes a difference if I want the same size wheel front and back.)
Anyway, I would guess that you may get by with maybe a 15x8 if it has about 5 1/4 backspace. But find a dealer who is willing to try several combinations on the car before you lay out the money. Mount them up and put on both front and back and check for problems. I ended up with 15x6 Torque Thrust and still 215/70R15 tires to gain a little more tire to rear fender clearance.