RE: Carbon Express Maxima 250's or 350's
I would have to agree that the 250s are probably just a touch light in the spine, however the 350 at the same length will be as stiff as rebar.
To be quite honest, what you should be considering is reducing the poundage of your bow to get the 250s to spine properly. A DXT at 67# would have more than enough "umph" to get the job done on whitetails. Besides, you don't want the animal to feel the hit like a truck. You want a super sharp broadhead and a perfectly flying arrow to cut cleanly in one side and right on out the other, with as little notification to the animal as possible.
As for the heavier arrow scenario, your KE will increase just a hair due to your bow being more efficent with a heavier arrow, however getting it tuned properly with a very stiff arrow can be challenging. If you wanted to shoot the 350, you will most likely end up having to leave them at least an inch longer than your normal arrow length just to get the spine down.