Chris,
Before you start, put the bike on the rear stand and a jack beneath the gearbox and gently wind it up until you see the weight coming off the springs; then try and find the sweet-spot where the top and bottom spindles are unloaded and thereby ready for removal of the spindles and spring-boxes. This applies to the std Vincent set-up.
One word of caution. If your bike was fitted with the long Thornton springs sometime in the past, they are under huge compression and there is no chance of getting the spring-boxes off safely without the use of a specially made-up tool to fit over the ends and take-up the compressive force. But if your bike still has the std Vincent damper, then chances are there are no Thornton springs in the boxes.
Be Safe,
Peter B