I've got the exact same rear. Got it from Sean many years ago. Finding 18" radials for stock front to match the 17" rear is getting tougher every year. I think you may have to look for a 17" conversion.
Congrats on your retirement! You're going to love it. I went a few years ago but would have stayed for a bit longer if not for the politics. I have many friends who did exactly that as well.
Remove your air filter and check that your needles are moving freely. I used a screw driver to reach in and move each slide. Of course be gentle. One needle was hanging up and causing a backfire as you describe.