j20 steering boxes....one more question

I've read over on pirate than only 1981+ j20 boxes will work, but read from bones here that 1980+ will work. What is the reason the earlier ones don't work...do the earlier ones just have a 3" piston, then were upgraded to a 3.5", or are there other reasons why they are said "not to work"?

So pretty much i'm asking, are all j20 boxes with a 3.5" piston created equally? thanks _nicko_