the 904 trans is shorter,you will need a longer one,you will also need the smaller 904 slip yoke.

IMHO,it is best to slide the yoke in till it bottoms out and measure from center of yoke to center of yoke then subtract 1" for best fit.

in my truck the shaft with 904 was at 54" with the 727 it is 52"

adding a couple inches to the shaft you have will give a ballpark measurement while looking for a ready to run shaft.