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It is important to get the right length of shock. Sorrry if I am going over things that you already know. You need to have 60% of the shock travel available for compression. So, set the frame height at normal ride height and then measure the distance between the upper and lower shock mounts to establish the shock length at ride height.
Most Cobras are around 2500-2700lbs. I would use a 500lbs spring in the rear. The length will depend on the shock length. You want to use as long a spring as possible to eliminate coil bind and you want there to be a little tension on the spring when the suspension is at full droop or use an axle limiting strap to prevent the spring from popping out of its seat.
For the shock length that you have specified you can use between an 8 and 10 inch spring.
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