Oh crikey, Zoidburg is right - you could end up with a
very bendy piece of wood.
Green wood shrinks, often substantially, and shrinkage is not even - it depends upon the grain direction, so it will shrink more in certain planes than others. This is one reason for splits and cracks in the first place - another is the straightening of the fibres, which often applies a torque to the piece as it dries.
One way out of your current position is to make the piece and get it in place, hoping that the fixings will hold it in shape whilst it dries. This is how green oak beams can be done, as large beams are often too thick to kiln dry anyway. Another possible way is PEG (polyethylene glycol) - cut and shape the piece and get it into a PEG solution before it dries. The glycol will substitute itself for the water in the wood. This might be expensive, but the wet wood shape will be retained with minimal shrinkage.
Preferred method of air drying would have been to plank the trunks and sticker-stack them with air gaps for about 5-10 years, planing them flat and square after that. I know, that's a bit like asking for directions and being told "I wouldn't start from here..."
Simplest and cheapest option might be to simply go for it, and take any twistiness as a feature, not a fault.