A DIRECT SWAP '71 904 for '70 904 plus... ???

The two 904 tranny's should be just a straight swap. If you put the 8.75" rear end in there, you'll need to shorten the driveshaft approximately 2.25" to compensate for the difference in the depth of the two rear ends - ie, the 8.75" pinion will be further forward than your current 7.25" (?). You will probably need to change the U-joint end on the driveshaft to fit the 8.75" pinion joint, and, the spring mounting plates will need to be changed to the 3" size for the 8.75" rear, as well as new / larger U bolts to go over the axle and into the mounting plates.