If you're keeping the 6, consider swapping in a T-5 for a good shot out of the hole and long legs for extended trips. You can do it for around $1500 and it will be ready forthe V8 when you are ready.

Just my $0.05 (It used to be $0.02, but now it's a nickel!)

 

If your classic stang is your DD, yeah, gas mileage is a big factor. My classic is gonna be the cruise car. something I take out to a nice resturant with the misses. The car I take on a road trip down Rt. 66. I don't look at my 66 as a DD. Even when it's finished and running like new, I still won't look at it as a DD so the mileage will be like a "convienance charge". By time the car is finished, I'll be making enough to drive a 10 MPG car as a DD. LOL!!!