1975 904 for slant 6

All ones listed are 11 or 10" but also again the converter for 318 & /6 can't be the same part ? Or can it ? Doesn't make sense. And for sure the 727 can't take the same converter as the 904 . Yet all these sites list it that way . I just want to be able to get the right parts and seams it shouldn't be this hard . Millions and millions of slant cars made