I think your tach is off ,or your speedometer is off. If the speedo's off, maybe so are your MPG caculations. Did you get the right speedo gear for the 700R4 to run 31's?
According to the gear calculator at
http://www.f-body.org/gears/ you are at 1700 RPMs on 31's doing 73 MPH.
You mentioned in another post that you are going into OD (.70 gear ratio) at around 40 MPH. At 43MPH in OD you'd be at 1000 RPM. I think your 3.08 gears (numerically low) have your engine lugging with he 31's.
You may want to drive in 3rd and manually shift into OD when you reach 70MPH. Or maybe try some shorter tires for a while if you have some available.
Your stock ECM /PROM are calibrated for 22# Injectors not 24#.
Fuel pressure on a 5.7L should be 35-39 psi. (42-46 is for 5.0L engine).
Maybe you're running rich from too high fuel pressure and too big injectors.
Another thing may be to check that the TV cable on the 700R4 is adjusted properly. Not only could maladjustment effect the when it shifts but will lead to premature death the tranny itself.
I'm new to TPI myself so the above info comes only from books and internet reading, not a lot of first-hand experience YET. But I'm learning alot chasing down a cold start issue (92% sure it's the cold start valve itself).