Flex plate

Since you can get the transmission separated pretty easy, take it back off, and then remove the flex plate from the motor. Line it up on the torque converter until all 4 flex plate to converter bolts will go in. While it is hanging there bolted to the converter, take some spray paint and hit one ear of the flex plate along with the converter. Take the flex plate back loose from the converter and bolt it back to the motor and torque it to 55 foot pounds. Reinstall your transmission to the engine and then line up the flex plate ear you painted to the converter mount that also got paint. Install your bolts that bolt the converter to the flex plate.
We all had to discover this the same way you have, and as you have found out, it pisses you off so bad you will never forget it! There is no locating pin and if I remember right, the flex plate to crank bolt holes won't line up if you try to bolt it on backwards. Good luck to you....you'll get it!