That doesn't look like a Sea Spray...it has all that storage on the hulls and what sounds like used to be a wooden platform instead of a trampoline. However, the span between the hulls is relatively small and the diameter of that beam large that I too would suspect that you do not need the dolphin striker. If you were to add one, I would suggest using aluminum bar like most other catmarans have. You will have an extremely difficult time getting carbon fiber to bond to the existing aluminum. However, considering that your mast pin doesn't extend beyond the beam, I'm quite sure the boat was never intended to have the dophin striker (truss) support there. You should be fine like it is.