Re: Optimal AR barrel length
The 16" will certainly work, but forget about the short/stiff argument with the ARs, it doesn't apply. The FAR bigger concern here is floating the barrel. a heavier barrel is certainly stiffer than a lighter one of the same length, but for accuracy, a float tube is the real ticket, with barrel weight coming in a distant second. Over the years, I've gotten very accustomed to the standard 20" barrel of my match guns, and feel that anything much longer tends to become unwieldly for many situations. The 20" is a nice compromise. Longer barrels will gain you a little velocity, shorter barrels will cost you some. Where that trade off suits your purposes, though, is ultimately up to you.
As far as the twist rate, tha 1x8" is going to be the most versatile for you. It'll still shoot lighter bullets just beautifully, and you'll be abnle to run anything up to the 80 or 82 grain match bullets for long range.
A float tube, a good trigger and a decent barrel, and you'll be ready to run.
Hope this helps,