3.42 to 2.73 Gas Mileage!

[HAULIN IT]

I have to agree with ladder, if you only drive 4-5 miles a day...by my math, you have been spending about $350 a year in gasoline. Even if you double the mileage (which you won't come close, driving mostly in town you will never even get in overdrive or lockup) it would 5+ years to come close to a transmission swap. Not to mention, you stated that the truck is only worth $1000 or so. 
Event is on the right track also...how you drive at each stop sign/light is free & could be just as important as anything else. Less gear will likely make you think you need to "give it more gas" to get yourself moving.  Lorne   

[Blazin]

The only reason that truck had any umph is because of the 283. When that blew up I dropped a sweet running, and probably the most powerful 305 Ive ever owned in it, it did lose a little in performance but the milage stayed about the same. That truck plowed great, it was only a 7.5' plow though. I think do to the high gears it didn't spin as easily. I had to use low range for a few of my driveways though. My two trucks I plow with now are both SM465s 86 K20, 350 with 3:42s, 8' plow about 10 to 12 MPG, and 86 K30 dump 454 with 4:10s, 9' plow about 6 or 7 MPG. My next plow truck I build will be a TH400.

[Edahall]

Although not worth it, if you're just driving short distances, the way to significantly increase your mileage in your truck would be to install an older 6.2L diesel engine with the small precombustion chamber heads.  The diesel engine doesn't go through the choke cycle when it's warming up so there's gain there and they are much more efficient than a gas engine when idling such as stop lights around town.  This engine would be the perfect engine for someone that does a lot of stop and go driving such as on a mail route.