Autocross/Road Course driving on 15" wheels. What's your experience?

Hi guys, I am looking for a little help. I recently installed a QA1 suspension on all four corners of my 1970 Duster 340. I am looking to take her to both track and autocross events. I also upgraded to the 5X4.5 bolt pattern all around. I am looking at 18x10 inch rims currently. Does anyone have any guidance on backspacing and offset? I saw two articles about Dusters doing the same thing running 18x10 and one was using a 25mm "backspacing", I believe they meant offset. one was running 315/30R18 all around and the other was running 275/35/18. Any guidance would be greatly appreciated.