[my67c20]

From your original post it sounds like you do occasionally hit the highway, but did not mention use of the truck, or a possible budget. Doing an automatic w/overdrive swap is not all that difficult. If you are not worried about removing the clutch pedal, having a neutral safety switch, back-up lights or a column shifter, it only gets easier. Its is easy enough to add a cooler, connect kickdown linkage, vacuum line and have the driveshaft modified. If you wanted to get a stock look, there are plenty of people parting these trucks to find all the parts needed for under $500. Recently swapped out the SM465 for a TH350, with 3:73 gearing. Probably could have completed it in a weekend, if I did not want to clean and paint everything. If automatic is not your cup of tea, then look at doing a NV3500 swap. Plenty of kits and posts about this change. Changing the rear gears only will get you some highway benefit, but will affect low speed drive-ability with the SM465, hence finding and overdrive. Another option, although more expensive, would be a gearvendors OD. Nice part of this option, is that if you ever wanted to sell the truck, just remove the unit and use on the next. These trucks are noisy and a 307 at 3000 rpm sounds like its gonna fall apart. Bottom line, you need to knock the RPM's down.