Mid Ohio, can be slick. I found to stiff a rear on my GT car was nerve racking. I hate push, but a car that wants to loop is no fun either. You are smart to ask, but as you have found, everyone has their own ideas. Here are two options. Take all the front spring rates and average them. Do the same for the rear. Start with these average rates. Second option would be to leave your fronts alone and start with the rear. You have a 275, rear now. Purchase some 400 rear springs and run the car. You will get an idea from this change how spring rates affect handling. In the end, it will come down to personal preference. If you can setup the suspension with the same free height springs front and rear, it makes it easier to swap springs front to rear to try different setups.