I have owned a 1972 Chevelle, a 1972 Nova and a 1969 Camaro a very long time ago. Most of those cars got around 15 MPG and all of mine were bone stock except the 1972 Nova had a lower rear end gear swap at some point before I bought it. Someone was trying to make that Nova faster off the line and it was. The 1969 Camaro had a 327 with an automatic trans and tall rear end axle gearing. I remember running 130 MPH and it was turning 4,000 RPM at that speed with plenty more to go. None of these cars got fantastic gas milage, but the Camaro got the best with it's tall rear end axle gearing.
When you tell me you want to modify your LT1 motor I am going to guess any dealer non stock parts available for that engine are mostly performance mods.
To me to save gas with that car is going to require getting the RPM's of the engine down while you are driving it. You are not wanting super low gearing in the rear end axle if you want better gas milage. Lower gears cause higher engine rpm at any road speed. Your car might be fine as is depending on what gearing is in it already.
You can change rear end axle gearing and rebuild transmissions to achieve whatever you want with a Chevy/Pontiac. Lots of things you can do with a Chevy product because there are tons of parts made for Chevy cars to change whatever you want about the car.
To make my post FUN I am going to suggest you check out a car built by a guy who goes by the name of Speedycop who races in the 24 Hours of LeMons races. They race $500.00 cars in that race. Check out the Speedycop Bonneville Donk description why the build got better gas milage and then scroll to the bottom of the page to see some of the other cars he built. Speedycop has built incredible cars for that race. Here is the link:
The Greatest 24 Hours of LeMons Cars of All Time - Roadkill