You need to re-write your question. Youd don't say what your current ratio is but it is likely 3.55. Also, you don't state what the D44 from the 78/79 is either and it is likely 3.50. But, it could be 4.1 or something close to a 3.07 or 3.08.
Go to the Tech forum and look at the stickies, especially the first one. You need to do a lot of research if you are planning a SAS if you don't know gearing basics yet.
LOOK AT YOUR TAG on the DIFF Are you ready to take on the task of an SAS your not playing with an erector set. From the looks of all your posts you may need to do ALOT of research before you just jump into this. IMO
You can use your existing but most people change both out unless you are a cheap ass like me. Im rebuilding a 9" so Im putting in stock gears till I put the 9" in.