Im going to be going with a 15x8 +10 with 175/55/15 I think.
Good on ya. That is exactly
what you need to make it work.
Im trying to decide if I should just go with a stiffer cut spring for the rear or actually shell out the $500 for a coilover setup.
IMO, Coilovers are ideal for those who want to change the ride height often, or for those who need to corner weight the car for track duty. For everyone else, static drop is where it's at. Dodge D50 truck front springs are great for the rear of a 510, firm but not punishing, cheap to buy, and already a known quantity.
You can cut those to a good height and get a 700-750 spring rate, which translates to a 185-200 wheel rate. You can also get regular 5" circle track springs in almost any rate you want for a few bucks more.
General rule of thumb for a 510 is to take the spring rate and divide by 3.8 to get the wheel rate. So an 800 pound spring in a 510 has a 210 pound wheel rate.