and how do i know which one i have....my car is a 1972 plymouth scamp with a 225 slant 6.

Well, the diff is the thing in the center of the rear axle...
Couldn't resist.
The difference is size and strength, 727 is big enough for big blocks, they were found behind a few slants, but it's rare to find one that way.
You've got a 904, still a great trans.
Pan shape is easy way to ID;
904 has square pan with only one corner "cut"
727 has corner cut and a kickout on the other side.

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