360 small block info

Hello All !! My 360 is, I believe, a 1970 model. When I removed the heads j code and ran the numbers, they came back as a 1970. My dilema is all the sites I have searched dont list a 360 in passenger cars till 1971. I assume that means my motor is a truck motor. No problem, but I am hoping someone can tell if the are both external balance motors. I am doing a tranny install (727) and will be using a B & M flex palte for the neutral balance convertor and for a external balance motor. I had a vibration in high gear and believe the tranny is the cuprit as I was also getting the loss of third gear under hard acceration (motor would rev until throtled back then would pick up third). The out going 727 had the flex plate for a neutral convertor, so I am assuming the set up was proper, but that is my concern. Is a 360 prior to 1971 a external balance motor ? Thanks !!