I'm in the midst of changing out my fan clutch on a 300e.

I pulled the radiator to make the job a bit easier. Made a tool to hold the pulley that worked great. Unfortunately I ended up rounding out the 8mm cap screw with a snap-on driver. The car has 210k miles and I think someone replaced the unit once already - and over tightened - as the clutch on the car is a Behr. Anyone have an idea what to do next?

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E55, 300E, 65 Hipo Mustang, 66 Shelby GT-350, 69 Mach I, 66 Fairlane GT Conv., etc