Welcome to the world of high-performance tailwheel aircraft. They can be a handful, especially something with narrow track landing gear like a Spitfire. Reducing prop RPM (note, this means prop pitch is increasing) will actually make it more difficult. Good practice is to land with the prop set for max RPM. Make sure you land with the centerline of the aircraft aligned with your direction of travel, and touch down at an appropriately slow airspeed (I forget what it is for that aircraft, but I'm pretty sure that info is included somewhere in the package). Once you're on the ground, stay ahead of any tendency to swerve. Making a small correction as soon as you see any deviation is much better than trying to make a larger correction a moment later, since you are much less likely to over-correct and induce a swerve in the opposite direction. If you do swerve off, or towards the edge of the runway, correct your course to parallel the runway centerline and get the airplane stabilized before trying to correct back to the centerline. Apply the brakes evenly. Usually it is better to correct swerves with the rudder rather than using the left or right brake. Also, when taxiing, stay slow - 10 mph or so. That might seem painfully slow, but realistically that's what you have to do to stay in control. Remember, you're still flying the airplane until the prop stops turning and the chocks are in!
Hopefully that helps.