I've searched exhaustively for the last hour, and I'm still not entirely sure of which flexplate my '69 Mach 1 used originally. It's a 351W with the original FMX automatic. So far, I
think it takes a 164-tooth flexplate with a 28 oz. balance. However, just when I've convinced myself that I've finally find the information, I come across another website that states something totally different. I don't have the original flexplate, and there are a myriad of options available from the parts stores. I just hate to buy the wrong thing!