I personally would recommend the 6.5-06 AI over the 6.5-284 and the 6.5 Gibbs. Not really for any performance reasons but mainly because it is far easier to load then the Gibbs and no harder then the 6.5-284 and offers a significant boost in performance over the 6.5-284 and also just plain feeds smoother.
With your controled round feed receiver, this may be an issue. Getting the short 6.5-284 to feed well in that Pre-64 receiver may take some doing, the 6.5-06 or 6.5-06 AI or 6.5 Gibbs will feed much smoother for you in your receiver.
Performance wise, I can not tell you on the Gibbs, never loaded for one. Built alot of 6.5-06 AIs and can tell you the 140 gr class bullets can easily be driven to 3075 to 3100 fps in a 26" barrel with top end loads. I load my personal rifle to 3050 fps with the 140 A-Max and leave it there because its a legit 3/8 moa rifle with that bullet at that velocity.
Another advantage of the 6.5-06 AI is how simple it is to load for. Here are the steps I use and recommend for forming 6.5-06 AI cases:
1. Buy some 25-06 brass
2. Prime that brass with a standard large rifle primer
3. Throw a moderate to top end powder charge for a 6.5-06
4. Seat a boattail 6.5mm bullet right into the unexpanded 25-06 case necks. No expanding is needed if you use a boattail bullet and debur the inside of the case mouth properly. You will not even feel a difference seating these slightly larger bullets in the 25-06 case.
5. Shoot, out pops a 6.5-06 AI case ready to be trimmed to length and loaded with full pressure loads. I have also yet to have a 6.5-06 AI rifle not play with that 1/2 moa accuracy level shooting fireforming loads.
No necking up needed in any way.
For those that feel the 25-06 is not a quality case to use for long range shooting, if you like you can short cases by weight, you can also take a light truing cut on the case necks to uniform their thickness but I have not had a problem. Winchester 25-06 cases are plenty strong enough for high pressure loading. Much harder in the case head then Rem cases.
You will get very close to a 264 Win Mag performance with this round and in a very easy to load for wildcat.
There is nothing wrong with the Gibbs either, just a bit more involved when it comes to case forming and you need to understand what your doing when you size your cases to get the shoulder location correct before you fireform your cases.
With the 6.5-06 AI in a rifle that is set up correctly, this is not an issue really.
Kirby Allen(50)