You would never get 16 out of any carb. gasser. My 6.2 on 35's with 4.10's and no o\d gets 25 or so on the highway. Thats the only thing the 6.2 was ever built to do was fuel economy. Dropping a 6.2 in it is almost as easy as dropping another gasser in it. You need to decide what you are wanting out of the truck. If you just drive it and want better fuel economy then drop a diesel in it. If you don't have money stick a 6.2 in it if you want more power and want to spend the money put a cummins in it. It's your truck.

 

I'll agree with that.

 

i have a 76 gmc jimmy (blazer) with the 350cid carb'd, 307 in the 12 bolt, the 256(i think) tranny, and the 205 t-case. i get about 12 - 15 mpg and i'm running 35' mtr's. it will easly do 85 down the highway without any problems.
i also have a 75 gmc 2500 with the 6.5 pre-durramax non turbo, it has the same tranny, same t-case,same tires, even started life as the same 350cid, the only difference is it has the 14ff( however i don't know the gearing, i'm thinking around 4.11's) but anyways it's getting about 20 - 30mpg. however it doesn't really like the highway and i don't dare push it past 65, it's screamming pretty loud at 55 and it's my first diesel with no tach so i just drive it slow for now
but good luck on your swap if you still decide to go with it
i'm currently looking to turbo it and probally regear it a little taller

 

I have a cucv m1008 I have been getting 16.8mpg j code standine pump not retarded
The truck runs great 6.2 from Alabama repower good and reliable starts with no glowplgs in summer I run 456 gears and when I cruse at 55 15mpg

 

I have another engine coming a 6.2 from the repower program if you run them on the hiway run 20 50 oil or if you want it to rn cooler and last Conoco Philips la 15 40 la synthetic made for landfill engines like the cat 3520