actually linkages are the same for both , the 727 is 4 " longer though , you need a new converter for the 904 , mount is the same as well .
the 8 3/4 is longer then the 8 1/4" as well so swap both at the same time or you will have to shorten the driveshaft twice , the axle tubes are 3 " on the 8 3/4 as well so you will need new shock plates , there are 2 driveshaft yolk sizes , 2 1/8" & 2 5/8 " so you need to match that up or get the conversion U joint to mate them up