Well,i'm not sure as far as a sbc,but have you ever considered a 4.3 v6? I had a 72 C-10 2wd 350/350,3.73 gears,and 27" tires (275-60-15),and swapped to a 30 over 4.3,marine hyd. roller,cast iron q-jet intake,milled out to accept an Edelbrock 600,home-built headers (very easy to do,use sbc truck headers),and a 2.71 or 2.73 12 bolt from a 69 Impala,and went from 15 mpg avg,to 20 or better.It was more labor than money,and the truck would still boogie pretty good around town.The biggest problem I had,was that the 4.3 sits so far back,there is no crossmember under the side motor mount bosses,so I fabbed vertical mounts off the front of the block like a 55-57 Bel-Air would have.Other than that,you need to extend your radiator hoses, p/s hoses,and fab up the headers,or find some v6 headers.If you have tall tires (30 inches+),the highway gear is gonna make it a real pig,but will certainly get good mileage.-Lew.
