What k Member do I need

I don't think you will contact the sway bar. I ran 15x7 rims with 4.25" backspacing on a 68 Barracuda with 68 sway bar and didn't rub with 225/60/15 tires. The car had been converted to large bolt pattern brakes with the appropiate 73-76 spindle and disks/hubs. That setup is supposedly a little wider than a 68 style small bolt pattern disk. But my tire were not close to rubbing with those tires.

Now that is a lot of tire for that setup and may rub your fenders. Some people have had issues.

Are you sure it's a 15" with 7 1/2" wide rim? That's sort of an oddball width. What rims are they?