A Body Tire Clearance
Absolute biggest you can go with a Valiant body and a 1/2” offset will be 275’s, and even that will probably require some quarter lip trimming/rolling. Backspacing will depend on on the width of the rear axle, if you do plan on customizing the width then just pick the wheels you want and set the width on the backspacing you can get.
Although I agree with Professor Fate, an 8.25” isn’t a bad choice with a 400 unless you’re talking about a big power build or lots of racing.