Tailshaft swapping an RV 727, What output shaft is correct?

Pretty sure 904 tail housing is shorter than 727. Then there are several different 727 versions. More details on your swap plan should be provided.
In general, staying with close model year to what you have, the output shaft and tailhousing can be swapped between different 727 transmissions.
I assume you are making a short housing from a 4x4 or van/camper into a long rear wheel drive housing for passenger car?
Here is a picture of an early 80's rear wheel drive truck 727. Fits and works fine in a car, but tail housing looks a little different.
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