Well here is what I have learned, which may or may not help you out with your decisions or not.
First off, just because you look at the manufacturers recommended twist doesn't necessarily always mean that a particular weight will not shoot good with a particular twist. Length has more to do with the accuracy than weight does. Granted you can have two identical weight bullets, but if one is a match type HP and the other is a lead tipped, the lead tipped one will be shorter and can usually be made to shoot over a wider range of twist, if that makes sense.
Another thing is, that just because you are putting in more powder, it doesn't necessarily mean your going to get more velocity due to it. That is a balancing act as with some powders the more you use the hotter they burn which results in a quicker rise in pressure. So if your using XYZ-2225 in a standard 22-250 and getting say 3350 form a particular amount, it doesn't necessarily mean you can just add more of it and get 3550 from the AI version. Usually you will have to go to a slightly slower burning powder to make up for the added volume to keep the peak pressures lower. Faster twist means more pressure, and it builds faster as well.
With my 25-06 AI with a 1-9 twist 5C Broughton, I quickly found that my standard powder RL-22 for the regular version was a bit fast when I got up over the loads I normally used. I couldn't reach the same velocities, without hitting pressure issues. However switching to Ramshot Magnum, I am able to get more than enough velocity without seeing the pressures spike up like they did with RL-22.
If and when you build your rifle, build it based upon a selection of weights and lengths, and try and pick a compromise. While you might be initially looking for a twist that will shoot the 70gr weights, you might also consider going slightly faster if possible. In other words you might find the 70gr bullet selection is a bit limited, but find that 80gr bullets have a wider selection of not only brands but also types. So if your on the ragged edge of getting the 70's to stabilize the 80's probably won't at all. On the flip side however, going to this faster twist, can with some bullets actually rip the jackets apart at the higher velocities, depending on how they are built, or based upon what type of rifling you have. A 3 groove will not be as easy on them as say a 4 or 5 groove, and standard rifling will be harder on them than say a canted land or polygonal. Just that some jackets are thinner and more fragile when considering the varmint type which are designed to virtually explode on impact. I have also seen this to be true when pushing conventional weight and caliber J-4 jacketed bullets to higher than normal velocities down a rough or tight twist barrel. Of course if you throw in some of the solid copper types like Barnes or the newer ones from other brands, then everything changes. However just because those type will shoot great doesn't mean the standard cup and core types will handle it. Like I said it is a balancing act, and only you can determine what your end goals will be.
This link will help out a lot with figuring out what might work best for you,
Barrel FAQ
Also before spending a dime, it will pay big to contact and speak with a couple of different barrel manufacturers about your plans and see what they recommend and why. Trust me when I say, they know what works and won't work, but they will build you anything you wish to spend your money on.
One last thing to consider, velocity isn't everything. Myself, I compromised on my twist to enable me to shoot a heavier custom made bullet which is longer and heavier than the standard 120gr heavy weights which are factory made. That said, had I gone with a standard 1-10 twist, I could probably reach another 100+fps before hitting any pressures that I see with the 1-9. Even so I still accomplished my goal with getting higher velocity than I could with my standard 25-06 using standard weight bullets. As it is, it shoots VERY flat out to over 350yds, and is 1/2 to 3/4" accurate out to 300yds with the 120gr bullets and the load I have worked up. I might should have gone with the standard twist, but I am not fretting over what I have in the least.
Hope this helps.