305 v8 or 250 I6 for better mpg

[6977chev]

my 69 right now has a 355 with some kind of milder cam in it, i dunno the exact specs thats the way i bought it. it has 882 heads and a edelbrock 600 carb. the carb has the same rods and jets it came with when i bought it. it has a 350 tranny with a mild shift kit and i beleive the rear en is 3:73 gears. right now i get about 6-8 mpg and thats about a good as i can get it no matter how i drive it it stays about the same. so im thinkin of maybe swapping in either a stock 305 v8 or 250 striaght 6 which i can get both for free. i would rebuild which ever one i bought. which one would be the better choice to stick in for beter economy. i would love swap in the sm465 i have laying around but i lack all the parts to do so. so for now it will keep the 350 tranny. which motor would be the best? thanks guys