If you have the standard carbine stock, buffer, etc. you probably can use it with the rifle length barrel.
As said before, headspace is set by the barrel and barrel extension so you shouldn't have to worry about it. Have you thought about just getting a 20" URG and selling your 16" URG?
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