With a low-profile, stiff-sidewall tire like Modcuda's, you can get away with 1/4" clear to the springs. A generic 50-series will be more tolerant of minimal clearances than a generic 60 or 70-series, so that needs to be accounted for. For a 275/50, I bet anything greater than 1/4" would be fine if you kept the tire pressure at 36 or so.
Clair