904 Transmission slipping/not engaging - new torque converter

1968 Barracuda with 904 behind 170 slant six. Custom torque converter (1967 nose, 1967 rear).

After finally getting this “temporary” donor engine in last month, I tried to test out the car and the transmission wouldn’t fully engage - felt like slipping very badly. Could move a little but obviously something very wrong. Limped up and down block and back to driveway. Did NOT have kickdown cable (Bouchillon) hooked up and assumed it was not fully engaging clutches without kickdown. Looked on FABO and saw “don’t drive without kickdown attached.” So I didn’t drive.

Yesterday finally got the kickdown rigged up and tried to take her out. Same thing. With any throttle she is slipping badly, only seeming to settle at or near idle. Still not really pulling.

What is happening?

Observations:
When installing torque converter, got it all the way into the transmission but when attaching flex plate, the torque converter needed to slide 1/4”-3/8” towards the front of the car to mate up with the holes on the flex plate. Called Dacco (who made TC) and representative told me should be no problem.

Other facts: this transmission has always been a bit funny.
- Kickdown has never worked and I have previously shifted “manually.” Car will try to start in 3rd if in Drive.
- Feels like light shift kit installed at some point.