Could i put a 2011 v6 engine in a body of the 1967 mustang?
It wouldn't be easy or cheap. You would have to replace alot of things;
You would need custom motor mounts
You would need to do significant fabrication to get it to fit (the DOHC V6 is much wider than any engine the 67 came with)
You would need new radiator
You would need a new transmission
You would have to do some fabrication to get the transmission to fit
You would need a new driveshaft
You would need a new steering rack (The 3.7L uses electric power steering)
You would need new accessories (alternator, A/C if you have it, etc.)
There will be issues with the engine's ECU as it'll expect input from sensors that that 67 doesn't have
You'll need a new fuel pump
You'll need a new fuel system ( the 3.7L uses a returnless style system)
You'll need a new exhaust
and I'm sure there's alot more stuff that I overlooked
In short, you're going to end up spending much more than the car is worth trying to get this work. Fuel mileage shouldn't even be a factor, I mean you're already spending around $15k as is, at that point an extra 3 or 4 MPG isn't going to make much a difference, compared to the money you've already spent. No offense intended, but I don't think you realize the difficulty and expense of what you are proposing.