The shots will be pretty tricky to be honest, and they'll only get trickier the longer focal length you use. We generally use 1/125 or 1/250 to show movement in the props. The golden rule for shutter speeds is to always use one which equates to 1/10th of the focal length, so on a 200mm lens you wouldn't want to dip too much below 1/250 hand-held unless you were supported. The tricky part however will be the helicopter, as they vibrate like hell.
I'd ask the helo pilot not to hover, but instead crawl forward at a slow pace. This will reduce the vibration in the cabin. Try not to use the sides or floor as support as these pick up the vibration of the rotors more than anything.
Before the spit flies past, try a couple of test exposures using a minimum shutter speed of 1/250s, either in Manual mode or Shutter priority. You don't want to go much faster than this as you'll freeze the motion in the prop and the aircraft will look like it's falling out of the sky in the images. Bear in mind that if you're shooting in RAW, you don't need to worry too much about the images looking flat on the LCD screen as you've got plenty of adjustability; the most important thing is that you catch the shot. Don't worry about exposure compensation or bracketing if you're in RAW either as you have +/- 2 stops either side of your exposure in PP, so just try and get somewhere near a 'correct' exposure.
Use continuous-servo focus (unless you're particularly comfortable with single-servo). Don't worry about your aperture, at the distance the Spitfire will be flying past (particularly if you're hoping to shoot at 200mm) then DoF won't play a big part, so you can shoot with the aperture wide open if needs be. Obviously you want the ISO as low as possible.
However, as I said earlier, the most important thing is that you catch the shot you want. If that means sacrificing motion in the prop to achieve a sharp shot then so be it.
Good luck, shooting from a helo isn't easy but it can be done, and it's bloody good fun!
