Will an LS1 get better gas mileage than my sb400?

[Hazieview]

I was wanting a way I could turn my '81 into a pretty reliable everyday driver that wont break me. The best route I can think of is to by something like an LS1 out of a camaro or something. You will still have the power but it will do better on gas than my sb400/350th setup. Right or no?

The way I see it my '81 is way lighter than a 98-01 camaro (i think)
These camaros get around 17 in town and at least 29 on the highway (I know because my girlfriend has one). So an engine with that much power in a lighter truck should do a little better right?

My whole reasoning for this is I like my truck and it is payed for. Spending 3-4 grand on an engine is way cheaper than buying a new truck that still sucks on gas and doesn't have as much power as I have now. Plus it's custom.

What do you think?

(by the way my 2008 crew cab chevy gets 18mpg bst 16.8mpg average...)