Be careful that you locate the rudder at or very near to the existing location. The balance of the boat is based on the relationship between the rudder, keel and sail plan.
What you're talking about is a significant alteration. There are probably other changes that would produce more "bang for the buck". A possibility is changing to a bulb keel. A deeper heavier keel would allow you to more easily carry full sail into higher wind ranges without requiring gorillas on the rail. Another possibility is to add a bowsprit and fly a true asymmetric spinnaker.
That said, there are many areas where incremental improvements might be had. But all at some expense without any guarantees. For the era it was designed, the J30 is an exceptional design. If the J30 is not what you want, then you're better off buying the boat you have in mind rather than modifying a proven performer. Besides, you'll shoot a hole in your resail value if your mods cant be backed out. This boat's value is in its one-design capabilities. People would rather sail one-design than go two-tenths of a knot faster.
