If you search around for posts on this here in the 500 forum you can see a lot of people have had this problem including myself...3 times. Here is the TSB. Let me know if I can help in anyway.
SHELBY GT500 - CLUTCH DRAG
2007-2009 Mustang ISSUE
Some 2007-2009 Mustang Shelby GT500 vehicles may exhibit the following clutch/transmission symptoms: hard to disengage or engage 1st and reverse, hard to shift any gear, gear grinding during shifts any gear, vehicle creeps with trans
in gear and clutch pedal fully depressed without brake pedal application, clutch slippage in gear with clutch pedal released, hiss noise with clutch pedal fully depressed (hiss on clutch pedal release is normal for GT500 clutch material). These symptoms may be due to flywheel distortion caused by excessive heat build-up during unique traffic conditions (example: severe stop/go urban driving). The distortion of the flywheel may prevent complete disengagement of the clutch. The system is designed for performance driving conditions. ACTION
Follow the Service Procedure steps to correct the condition. SERVICE PROCEDURE
NOTE THIS ARTICLE APPLIES TO VEHICLES WITH PRIOR REPAIRS, AS WELL AS THOSE WITH MINOR MODIFICATIONS, EXHIBITING ABOVE SYMPTOMS. NOTE THE GT500 CLUTCH SHOULD BE DIAGNOSED/REMOVED/REPLACED AS A COMPLETE ASSEMBLY ONLY. DO NOT TAKE CLUTCH ASSEMBLY APART. 1. Remove, drain, disassemble and clean the transmission per Workshop Manual (WSM), Section 308-03C. 2. Remove and replace input shaft and all synchronizer assemblies per WSM, Section 308-03C. 3. Assemble the transmission per WSM, Section 308-03C. 4. Remove clutch disc and plate assembly and pilot bearing per WSM, Section 308-01. 5. Remove and replace flywheel per WSM, Section 303-01C. Discard old bolts and use bolts provided in flywheel kit. 6. Install new pilot bearing per WSM, Section 308-01. 7. Install new clutch disc and plate assembly per online WSM, Section 308-01 (vehicles with solid design flywheel). Discard old bolts and use bolts provided in clutch kit. Tighten bolts to 89 lb-in (10 N.m) in a star pattern. Tighten an additional 90 degrees in a star pattern. 8. Apply a small amount of Motorcraft(R) PTFE Lubricant to the clutch hub splines per WSM, Section 308-01. 9. Install the transmission per WSM, Section 308-03C.
WARRANTY STATUS: Eligible Under Provisions Of New Vehicle Limited Warranty Coverage
IMPORTANT : Warranty coverage limits/policies are not altered by a TSB. Warranty coverage limits are determined by the identified causal part. OPERATION DESCRIPTION TIME