Wondering how much tire I can stuff under the back of my 88 vert with the stock BBS 15" rims. Can I go to a wider 50 series and stay with a correct tire height?

 

205 is the widest that will work well. If you wanted to prove a point, you could find some 275's to put on there. Tires, whether stretched on too wide or too narrow of wheels will negatively affect the best possible traction the tire could offer. Some will say you can do 215's, 225's, 235's, so on and so forth. If you really want more tire back there, just do it right and get wider wheels. Otherwise, stick with the stock size and choose a good set of tires. A good set of tires that's the same size as a cheap set is a night and day difference. Size isn't always everything, you know?