[A.J.'s Superior Automotive]

Klaus,
I've seen this happen many times when people install Automatic transmisions. Mostly caused by the torque converter not being completly seated into the trans. Some times this is easyer said then done. The lugs or splines need to line up. I've also seen burrs on the input shaft and in the torque converter. Be paitent and Don't seat it with a Hammer.
On the other end of the torque converter is a nub that needs to fit into the crank shaft. I've also added a light amount of lube on the nub when things get tight.

I don't know about a spacer plate. I don't have one in my '61.