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I’m not familiar with the Spitfire setup but if like some others it will have a built in antenna tuner which is adjusted by the DIP switches you referred too.
If that’s the case putting the antenna whip at the end of a piece of coax will probably not work very well. The tuner will be designed to match the 3 meter whip which is not a 50 or 75 ohm load. Putting the 50 or 75 ohm cable between the tuner and the antenna whip would present a big mismatch.
I’m sure someone more familiar with the Spitfire will chime in…