Fuel economy slowly improving after rebuild

[1SG.Rob]

Good morning! I recently had a stock rebuild done on my 396. When I first got it back from the shop, I was appalled at how bad the gas mileage was. I know these trucks aren't known for great mileage and that's not the reason we drive them anyway, but I was barely getting 100 miles out of a tank. However, the mileage has been slowly improving without me doing anything to the engine. I have about 600 miles on it now and am getting about 10 mpg. Is this normal? Does a rebuild improve on mileage as it progresses through the break in period? I haven't really changed my driving style that I know of, I drive pretty conservatively in the truck.