I had pretty much the same thing happen on '66 with the 200 6 cylinder and auto
tranny. One of the bolts to the torque converter was a little loose starting a tiny crack that spread and worked its way to the other bolt holes......Somehow I blew a chunk out of the passenger side of the C4
tranny & when I replaced it is when I found the cracked flex plate, part of the cracks were really sharp edged as if pretty fresh and some of the crack was smoother like it had been going on for some time, If the
tranny hadn't broke the flex plate would have gotten me.
Stuff happens........................................... ......
TJ