When I built a Triumph based Marlin many years ago, I used Spitfire front and rear springs. The fronts were shortened (1 coil?) and the rear had one leaf removed. Use the later "swing spring" rear from the MK IV spitty if you can - it reduces the risk of the famous tuck in on hard cornering. Also don't get carried away removing leaves from the rear as the spring tends to settle a lot after a few miles.
Robin
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