Actually the 15" crosslace rims from a Grand Prix are bigger than the 16x7s put on the Bonnevilles. The Grand Prix crosslace wheels are 16"x8" so they are wider (not taller). They never had any 15" crosslace except on the Fiero which has a much smaller bolt pattern.
Sad to say is you may have to wait until your Goodyear Regattas (NIce choice of tire -my favorite all season passenger tire) are worn out, or else you could just save the Goodyear Regattas for winter (that way you can have nice Regattas when it'* snow time).
Be sure you go to www.car-part.com
and search the New York salvage yards. This website has so many salvage yards connected that you could probably find some nice gold crosslace rims for dirt cheap.
BTW, by all means get the gold crosslace wheels. There is a maroon 92-95 Bonneville I see every once in a while and it has the Gold crosslace wheels on it. That car looks like it gets detailed every other day. It is probably the nicest Bonneville I have ever laid eyes on (that wasn't modifed other than the rims). The gold rims just make it look like it should be put up on a pedestal.
And nice job with the spoiler. Definitely brings the car to life.