You will be able to tell by eye when spinning the wheel. I've run wheels / tires that wobbled 1/8" laterally on a cars less isolated than an SL with no ill affects so you will be looking for something drastic. A square block can be balanced but it won't roll so well.

You can still have a driveline imbalance. Remember, the drive shaft spins about 3x wheel speed. If you are in direct drive ( top gear not overdrive. ) the drive shaft spins engine speed, if overdrive some % over engine speed.