Sorry, when I said "economically" I meant wallet-wise.

But to answer your question:

If a 2001 Z28 Camaro has a curb weight of approx 3600 lbs, and our VX is approx 4000 lbs... and the Z28 has an LS1 (5.7) with commonly boasted 26ish mpg (other forums)..... then I estimate about 21-25 mpg with a 5.3L in the VX.

That's just pure speculation using sorta-similar variables. The real way to find out is to ask the other forum members that have DONE it, like Wisdom.
WISDOM, PLEASE INSERT YOUR EXPERIENCE!