JonM's right, there are so many incompatible AR-10 versions you have to find out what you have. Now if you actually have a regular A2 stock and receiver extension you can get an A1 stock and lose the spacer that's required for A2. That will shorten it about 3/4". You probably can go to a carbine RE and buffer then add a Sully or Entry stock which is much shorter (if you want to stay with a fixed stock).
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together. You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek