I'm not sure why someone recommended that particular combination for better mileage. In my experience the mid 70's vehicles were a mixture of somewhat larger engines (350's), lower compression, and an attempt to satisfy newly imposed smog requirements. None of this led to better mileage. The best way to obtain better mileage is to install an overdrive transmission of some type, 700R4/2004R automatics, original 3 speed overdrives or the later T5's. There are also rear end options. Of course engine condition and tune up are always important. These trucks are heavy and push a lot of wind so they will never be truly economical.
Fred
