I'm thinking that adding an additional resonator to your exhaust will probably be the most effective fix for your issue for the lowest cost. I have a custom exhaust on my 95 SR and while it's definitely louder than stock, it's not overly annoying at highway speeds. Any transmission conversion you attempt is going to be hundreds and probably thousands of dollars. A decent exhaust shop should be able to recommend a resonator that can be added to your exhaust that will help eliminate some of your noise without affecting the performance for a fraction of the price of a transmission conversion.