That all depends on several factors some of which I will likely neglect to mention...
1. What distances will you likely be shooting at and what type of game? (More Length = More Velocity)
Game - 300 yards
Steel targets - as far as I can
2. Have you picked a particular bullet yet and if so what velocity do you expect to shoot it at?
No, but it will be 140 grain
3. Will you be carrying this rifle or will it be shot strictly from a rest or something in between?
Strictly deer stand or rest
4. Are you dealing with weight issues or is that not a problem?
OK... Enough information... I've heard people say that for a can in a hunting rifle that 16" is enough length and I beg to differ.
IMO you need to go with 20-22" in order to get reasonable hunting velocities out to 300 but some opinions might differ.
If you're serious about distance I'd go with 22-24" with 24" being the preferred length but I've found that 308 based cartridges typically get pretty good velocity from 20" barrels.
You're going to have to determine how long the rifle will be with the can on it and if you're willing to put up with that length and possibly front heavy situation and go from there....
So in summation here's my opinion. If it was for me I'd go with a 20" barrel plus the can and by the way that's my choice for a fat barreled 308 I'll be putting a can on. I don't care about shooting that rifle beyond 600 because I have rifles better suited for that although this rifle is crazy accurate.
Good luck and let us know what you do and whether you feel it was the right choice after getting it all set up and shooting it.
I would go with a 1:8 twist barrel regardless of length. And I would go 20-26 or somewhere in-between. 20" from lug to crown makes for a nice compact hunter/LR target gun. I had a custom .308 Win built on a Rem Model 7 action with a heavy barrel cut 20" from lug to crown, and it was an awesome toter in the woods. The short barrel didn't get hung up on limbs and crap. It was easy to get in and our of tree stands and shooting houses. It was easy to tote, and fairly lightweight, but still 1000 yard capable.
i am having a custom 7mm-08 built and have already ordered a heavy bartlein barrel. What length should I have the barrel cut since I will be putting a can on it?