Well if they are spined for 64 lbs at 26 inches shooting them at 70 lbs they will most likely be weak. Which is not good. Slightly stiff is ok, but weak is always less forgiving. Will the bow shoot them, sure it will. You may not even notice a difference with field points, but if you try to tune it with fixed blades you will most likely have some problems.

Just go the next size up on your arrow selection and you should be ok.

What type of arrows are we talking about, and what model of bow?

Paul