SFI Flex Plate differences

If you have the one with three converter bolt patterns, it should be the larger diameter flex plate that was originally intended for 727 applications. The smaller bolt hole converter pattern is out further from the center. That is for the really big 727 converters usually in big cars that are like 11 3/4 or 12 inches in diameter.
The 904 intended flex plates are smaller in diameter so there is no room for that larger converter pattern.
If your engine is balanced to your flex plate, you can put any trans and converter package behind it, 904 or 727, as long as you verify the starter will clear everything on a 904 setup, and as long as the converter is neutral balance.