Production GT350 T-10s were different than "production" Mustangs . It was "close" to the K code trans but had different 2nd and third gear ratios. To do this the GT350 trans used a different input , cluster, third and second gears. The other parts were the same except the '65 , 66 Crryovers and early '66s that had a cast aluminum case and tail housing or combination of iron and aluminum.Later (maybe around 1,000) units were all cast iron cases. In road racing , the cast aluminum cases often cracked and iron replacements were often substituted.