Do what you want to do, afterall it is your car. FYI though, plenty of companies now make 9" wide rims...it may be a nice way to 'bridge the gap' so to speak
I've got 17 x 11 Smoothies on the back of my '66 vert...but that is all under the sheetmetal [and not on the car at present...MUCH work to do]. Basically I just have this DEEEEEP looking wheel w/ narrowed 9" rear.