I just bought a 1968 chevelle I know that it is not original the vin is 136378a161815 i would to find out what it was supposed to have . and help

1968 was a transition year at GM. The were reorganizing the assembly operations under the GMAD and you will not find a lot of info on trim tags or in the VIN on Chevy models anymore.

136378A161815
1 - Chevrolet Division
36 - Malibu V8 (note-SS 396 would be 38)
37 - 2 dr sport coupe
8 - model year
A - Atlanta final assembly
161815 - production serial number which started at 100001 and went up making this the 61,815 Chevelle model off the Atlanta line (inc all models in the Chevelle line)