What year and cubic inch is the trans behind? Early small block had small rear crank registers. Thus, your convertor hub gets "jammed" into the back of the crank.
I will have to check, it's a 318 and off the top of my head I thought it was a 73 block. The converter freely spun all the way up until I mated the trans to the block and THEN it didn't want to spin by hand anymore. Which is why I figured the converter was in all the way and that was just normal. When I ran the part number on the crank it came back with the crank that was used in both the 318 and the 340 for a very short period of time. Not sure if that is in fact my problem or not. Maybe the hub is just big enough to bind inside the crank?