Re: how long of a barrel do i need ?
I'm a fan of long barrels and have seen the elevated velocity they produce.
All my rifles are at least 30" and out to 37". Theres NO WAY I could achieve the speed I have from my 338/416 (300 Gr bullets to 3310FPS) unless the barrel was at least 36".
Same with the 30/378 Weatherbys I have had (36") or the 6.5/300 Weatherbys or 7/300 Weatherbys with their 30" tubes.
All of my wife and my 1000 yard guns have 30" barrels on them.
I am ready to go with a 44" tube on a 300 Tomahawk so you see, I like length on barrels.
In ANY caliber you should see a 25 to 35 FPS increase per inch of additional barrel up to a point and that's starting at a 24" to 26". You can never go wrong with at least a 30" barrel.
Again longer is better. I like them 36" and more.
I'm not sure what you would get with the barrel and caliber you mentioned but, I would go with the full length to start with. If it was not satisfactory, you could always have your smith cut it off to a shorter length.
Remember, 100 to 200 FPS additional velocity means alot downrange.
Good Luck to you.