That would make sense for a stock suspension, although that is a tad small for the rear. I personally think you can go woder than that, even with leafs int eh stock location, but wheel choice becomes critical.
They have put 275 front and 335 rear using the Level 2 system under a Challenger. Not sure of the wheel size they are using, but it is in here somewhere. You can search for it by looking for posts from dqhemi.
Bump steer on their rack system is minimal. Since it is a completely fabricated system with relocated pick up points, they wouldn't have the same interference issue as a rack retrofited to a stock suspension.