Some general points to consider
A storm jib:0 should be not more than 50% luff length. If you have wire halayrds, you will need a head pennant which will allow the pennant top to be close to the masthead sheaves so the halyard tail ends up in the right place near the winch. If you have converted to rope halyards, this is not so important. While you want a fairly high clew to keep the sail out of the big waves which come with big breeze, you need to be sure that the clew height (when the sail is fully hoisted) is such that you can get a proper lead to the genoa tracks-if the clew is not high enough, even with the car all the way forward you will find the lead is too far aft for proper sail trim (and the top of the sail will always be luffing with the foot overly flat). The foot length should be about 40-50% of "J"-not more than 50%.
If you find used sail which you think is a good candidate but the clew is not high enough, you can add a short pennant to the tack to raise the tack (and along with it-the clew!). Use a rope while you are experimenting with the clew height and try different lengths until you get to a point where you can trim the sail to get an even break on the luff (using at least one upper and one lower set of telltales) with the car about 1 foot aft of full forward. This way you can still move the car forward for reaching and running with the sheets eased to keep the entire sail working. The limiting thing would be that you should not have the tack be more than about the height of the bow pulpit. That should give you about 2 feet of vertical adjustment between the normal tack fitting and the top of the pulpit to adjust a used sail to get the clew height where you need it. Once the tack pennant length (if needed) is determined, make it permanent.
Hope this helps!
