switching 14" axels to 15"

It's true a wider tire on the back will move the balance towards understeer but our old cars are already have extreme understeer where you will completely lose traction with the front wheels without losing the backs so wider tires aren't going to change that but our cars, especially with v8's, have traction issues that a wider tire can help with. You can make a much more balanced car with a wider tire on the back.

FWIW, I have 225/60-15 (7" wide rims) on the front and 255/60-15 (8" wide rims) on the back of my 68 A-body Barracuda. I used to have 235/60-14's on all 4 corners. I can drive much harder out of corners now with the bigger rears without fear of power induced oversteer. And, I don't have anymore or less understeer going into or through corners. Plus there is a big improvement in straight line traction with the wider rear tire.