If you have done excessive head work on a spitfire engine and discover that the stock (European style) dual valve springs are not long enough to fit the modified head, it has to do with enlarging the valve seats. In doing so, the material that is removed makes the valves sit deeper in the head and the stem sticks further out the top of the head.
While it is possible to shim up the springs at the base, if you are using a high lift cam, it may cause the sprngs to bind and therefore lead to cam wear. "Iskenderian" makes a set of racing springs that solves the problem. It is their part number VWE-005/SP-006. Depending on the amount of metal that has been removed from the head, shimming may still be required, but the springs are long enough that they will not bind even using high lift racing cams.