There are no brackets that I know of the convert the 5.25" to a 6.5" but you can modify the stock brackets to fit the larger speakers. I did this with my car with good results but it depends a lot on your speakers. I know a lot of people on CGP have made their own custom brackets for the larger speakers.
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1998 Pontiac Grand Prix GT
WizAired CAI, Impala 12" Brake Upgrade, ZZP Alternator Rewire and Voltage Booster, Rotated Dogbone Mounts, Kicker KS690 6x9s, JBL GTO427 4s, JBL P652S 6.5s
2001 Ford F150 XLT SuperCab
Stock, LineX, 212K