Changing from 7 1/4" to 8 3/4" rear end questions

I have an 8 3/4" rear end out of a 69 barracuda I want to put in my 69 barracuda with a small block and 904 transmission. I have the original sbm/904/ 7 1/4" driveshaft it measures 51 3/8" center to center. I saw in another thread that the driveshaft should be 49.27" are the driveshaft lengths different between 7 1/4" and 8 3/4"? Also is the 7 1/4" u-joint the same as 8 3/4"? Thank you.