I had a similar condition many years ago with a convertor that was to tight in my '64. You could floor it from a dead stop and it would almost shut-off the car. Difference is I had already had the motor dialed in but broke my good trans and it sent **** through the convertor. So this was a trans I threw in with an unknown convertor just so I could go to shows until the new one was rebuilt and the convertor cleaned. I played with the secondary pump cam and squirter which barely helped. Everything was back to normal a few weeks later when the new trans went in and the convertor was flushed. Just posting cause I don't think your thinking is out of line.