Found some good information, 2.5" was superseded in March 68 by the 2". Same case was used right through just different clips and pipe seal.
3015134 - Case - 1968-69 Camaro (except 396 engine) without AC
*use CI Part # 3013319 for 1968 Early Builds with 2-1/2" core
*use CI Part # 3011432 for 1968 Late Builds with 2" core (supersession #3016842)
*use CI Part # 3011432 for 1969 models
3015141 - Case - 1968-69 Camaro (with 396 engine) without AC
*use CI Part # 3011477 for 1968 models
*use CI Part # 3018864 for 1969 models
3015606 - Case - 1968 Camaro with AC
*use CI Part # 3009917 for 1968 models
3018763 - Case - 1969 Camaro with AC
*use CI Part # 3018489 for 1969 models