Trimming a spinnaker from a 39' Gulfstar to fit a Catalina 30 would be quite a job. Would you want to cut the extra sailcloth out of the middle, in order to keep the reinforcements at the clews and head intact? Is the sail made with parallel luffs (not tapered or rounded) so you might be able to remove one or more sets of panels so that the height would fit? If the sail is tapered or round in overall shape, this would be mighty tricky to do. Getting the foot length right would be another issue, possibly requiring darts, and the whole thing, after days and days of work, might blow out the first time it's set. It would seem much easier to tade in the larger sails, as already suggested, and actually get a sail that was made to fit a Catalina 30. There are lots of Catalina 30's out there, and probably LOTS of used spinnakers to choose from.