gt350 was made up in to 1970. i doubt its a engine from a shelby i had a idiot try to sell me a 429cj as a boss 429 i laughed in his face. or the normal 302 as a boss 302. most are trying to make quick money on a sucker. j code is a j in the vin number on the car the casting numbers are the same as a normal.only true way to tell is to fully strip the engine get the casting numbers off every part and check them on a shelby site.