I used run a 10" wide slick on a Corvette rally (15x8) on my '74 Chevelle.
The frame is the limiting factor. Massage the rail a bit and you can put a 13.5" wide tire on there with no changes to the wheelhouse.
The ones in the photos below are 11.5" on a 10" wide rim.
K
__________________
'63 LeMans Convertible
'63 Grand Prix
'65 GTO - original, unrestored, Dad was original owner, 5000 original mile Royal Pontiac factory racer
'74 Chevelle - original owner, 9.85 @ 136 mph best
http://www.superchevy.com/features/s...hevy-chevelle/
My Pontiac Story: http://forums.maxperformanceinc.com/...d.php?t=560524
"Intro from an old Assembly Plant Guy":http://67-72chevytrucks.com/vboard/s...d.php?t=342926