I have a NBS 88' 2500 with a 91 SS 454 w/cam and eld. manifold and fuel injected.
I run a TH350 w/GVO and 4:11 I can get 13 mpg with 225/60/16 wheels.

350 will be the same with a OD tranny for mileage cruising on the hwy.
BUT no power like the 454 .....

The truck came with a 350 TBI, I swapped it for a 350 TPI and then did the 454ss
Ron

13 mpg? Man I feel gypped! My 2wd 93 extended cab with a 454, 4L80E, 4.10 gears ridding on 225/75-16s gets between 9 - 10 mpg empty, loaded or hauling a 35 foot fifth wheel. The 454 is strong as hell will pull anything I throw at it, but can't under stand why it won't get any better mileage empty. My 93 runs good, passes smog with flying colors, don't burn oil, leak or even get over 210 degrees when towing heavy loads up hills. I picked up a crew cab dually a few weeks ago, threw it on a buddy's about 2,200 pound car trailer to bring it home & still got 9 mpg. If I got 13 mpg I would drive it daily.

my gas millage is 10$ a day
i had a 350 in my caprice and only got 8 mpg

i just pulled out my bbc and put a full roller sbc and it still rolles down the road just fine dont hit the gas any harder i have a 400 trans at this time i have a 700r4 that is going in it so im hopeing 4a lil better milege out of it. it is a 85 4door long bed dually