In days gone by, I've driven a truck with a "two speed axle", my Jag MKII with the Laycock De Normanville overdrive (very nice) and my Willys with a Chev small block and Borg-Warner overdrive. I've looked through some of the 6-speed threads here and Googled overdrive and six-speed transmissions on the www.

I really haven't found any realistic way of getting another overdrive gear. My GenII has headers and a tuned exhaust. It makes so much noise at 3300 RPM, I can't hear myself think. It would be nice to drop back 3-400 RPM.

Maybe I should just get a lot of sound insulation.

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.

I was afraid it's unrealistic. Both of my 3Ls feel like they could handle another gear at speed. Fortunately my Raider just has a glass pack, so it's not really to bad at high speed.

My dad has a '96 Corvette Collector's Edition with the LT4 and a B&B exhaust system. The resonant drone from the exhaust at 1500-2000 RPM is really obnoxious, and it's right at the 70-75 mph mark in 6th gear which is the typical highway cruising speed. He's been talking for years about replacing the exhaust or mufflers to eliminate the noise, but the exhaust sounds so good at all other speeds that he can't bring himself to change it.

What are your axle ratios? Maybe step up to a larger tire to bring your highway RPM's down

you could switch to the 4.22 or 4.27 axle ratio's depending on what axles your ride has it will determine which of the two and either ratio can be had at a junkyard out of a mitsu pickup in oz. here in the us you can get the 4.27's out of a blister fender montero and even some montero sports. 4.22's out of mitsu v6 pickup's. engine grut and depending on what type of wheeling you do you may end up needing to regear your transfer case which isn't a bad idea anyway.