1- Will the V8 get better gas mileage?

Possibly, but it would be a minimal increase if any.

2- Will putting a V8 in reduce resale value?

Yup. Guys like me wouldn't even consider buying it. 5.4L Excursions are rare, and most don't really want one.

3- Any other thoughts on this swap?

Yup. How'd this one break? How many miles?