X2 on the MY6, however the shifter will be moved toward the rear about 9 inches. An NV4500 5 speed swap will not move the shifter back as far, but is probably overkill.
OK, Transmissions with overdrive work great with the stock gears. If you replace your rearend with something in the 2.73:1 to 3.38:1, the OD transmission will not be necessary. In fact that sm420 4 speed is likely the best transmission available to use ig you put freeway gears in, as you can now use that granny low to get you going.
Either figure out which rearend you are going to use and select a transmission to match, or figure out which transmission you will use and select a rear to match. By the way, I'm not sure by what you mean a "better gear ratio". The better gear ratio depends on how you are using your truck. If you want to pull some loads, lower gears will be better. If you want to cruise the highway, higher gears will be better. If you want to do both, the factory gears with an OD transmission will be the best. It really depends on how you want to use your truck.
On the HO33 rear axle housing... I believe that axle is plenty strong for a 235. I would not put one behind any engine that makes over 300 HP, as this cast housing will not withstand heavy torque forces. I wrecked a completely overhauled HO33 in my 59 gmc with a 300+ HP motor in less than 30 miles. The axle perches were ripped right off the axle tubes, causing the tubes to shatter. Expensive mistake, but now I have posi!
