1990 454 SS BAD A-- TRUCK

I HAVE A 90 CHEVY 454 SS TRUCK AND SHE RUNS WELL 120,000 MILES AND STILL FLYS , QUESTION IS I HAVE A PROBLEM WITH A BOLT THAT HOLDS THE AC COMPRESSOR BRACKET, IT BREAKS OFF ALOT ITS EASY TO FIX BUT WHEN THIS HAPPENS I GET A VIBRATION OF COURSE.I HAVE CHANGED THE CRANK PULLEY IT LOOKED IFIE AND PUT A NEW BELT ON BUT AT SPEEDS AROUND 70-85 I FEEL A UP AND DOWN VIBRATION THAT TELLS ME SOMETHING IS NOT TRUE. COULD A BALANCER DO THAT.I HAVE BOUGHT ALL THE BRACKETS NEW 230.00 FOR ALL FROM THE DEALER AND THE AC ONE IS THE REAL THICK UPGRADE, AND TODAY BOUGHT A NEW TENSHION FOR THE BELT. ANY HELP WOULD BE GREAT THANKS JIM