Honestly I think you should use 91 or 93 octane, whatever is offered in your area. It spent cost that much more to run the better gas. If you have a 15 gallon tank and gas cost 3$ for 87 and 3.25$ for 91/93 octane, it would only cost you another 3.75$ to fill up from empty. That's not much IMO to run a much better fuel in my car.
I had a 76 Chevy pickup that my bro had given me. It had mid leanth headers on it. I replaced the carburetor and put long tube headers on it. When I removed the old headers each of the exhaust ports had a lot of carbon build up in them. When I put the new headers on I ran nothing but 93 in it for 6 months and one day I decided to pull a headed just to see if the car on was still there. When I got the header off they were all clean and I've used 93 every since for anything I drive. I seen first hand what better gas did.
You can't lose if you put God first!
1989 LX, 306 with TF top end kit, Anderson N-21 cam, TKO 600 trans with steel splatter shield,full Maximum Motorsports Mustang maximum Grip box, Detroit locker rear end with 31 spline Moser axles. Staggered wheels with Nitto 555 315/35/17 out back.