I believe it was Joe who told me I could do this. After I took apart and reassembled my 66, I thought the upper tension was very questionable. I was thinking about replacing the whole assembly. I decided to replace my older tension assembly with one of the newer numbered tension assemblies. Joe said it would work because the casting didn't change, only the upper tension assembly was different. I've ordered one and I'm waiting for it to arrive.
The upper tension on my machine wouldn't hold. It would sew perfectly for a while and then fail.