If you prefer a Chevy engine in your Cobra - go for it. Getting the six-speed tranny to fit is a challenge, but can be done, depending on what kit you get.
If you want a Ford, how about a fuel injected 351W stroker? There are several guys at
The Corral that are claiming to get in the 20s for gas milage with 400-500hp fuel injected 351W strokers and a 5-speed tranny.
Pete