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Spacers: Many people find that they need spacers to avoid rubbing against the upper control arm (UCA). A common size for spacers is 1.25 to 1.5 inches, but this can vary depending on the exact tire width and offset of the wheels. Tire Width: The most common width for 35-inch tires is 12.5 inches (35x12.5R20). This width often requires spacers to prevent rubbing. Some have opted for slightly narrower tires to avoid the need for spacers, but 12.5 inches is the standard choice. Trimming: Even with spacers, you might need to do some minor trimming of the inner fender liners and possibly the front bumper to avoid rubbing during full turns or under suspension compression. Alignment: After fitting larger tires, getting a proper alignment is crucial to avoid uneven tire wear and to ensure the best handling characteristics.

There have been some discussions in automotive forums about roof panel flexing in various truck models, but it doesn't appear to be a widespread topic, which might be why you haven't found much online. Contact the Manufacturer: Reach out to GMC's customer service or technical support to report the issue. Given your background, your insights might be taken seriously, and they might provide some explanation or solution. Check with Dealerships: Visit a GMC dealership and have a service technician inspect the vehicle. They might have encountered similar complaints and could offer a solution or at least document the issue. Join Online Communities: Join forums or social media groups dedicated to GMC Sierra or Silverado owners. Posting your experience there might help you find others with the same issue and gather more information. File a Complaint: If you believe this issue could lead to safety concerns, consider filing a complaint with the National Highway Traffic Safety Administration (NHTSA). This could prompt an investigation if the problem is found to be more widespread.