Hi Kevin,
Even with the 13.9" units, a severe jolt that completely compresses the front units (including the rubber bump stops) can cause the headlamp to leave a mark on the front mudguard (and crack the headlamp glass!). If you check out the front units, you will see that they have a longer alloy spacer on the piston rod, to control the compressed length. You may get away with the 13.4" units providing they don't have a shorter compressed length. Some modern units have top fittings that can be unscrewed - in which case, a spacer can be fitted over the piston rod to maintain the original compressed length. Also, the original springs on the front of the Dragonfly were the lightest ones listed by Girling, and rated at 54lbs/inch - this means the damping will have to be correspondingly light - otherwise the units may not recover before hitting the next bump - and will jack themselves down.
Regards,
Eddie.