Seems like, other than the cool factor, it probably wouldn't be worth it.
Say you get 40 MPG out of it when the stock setup would get you 20 MPG. Say gas is $2.50 a gallon. Say diesel is $2.75 a gallon. Say you drive 200 miles a week. Say you get the donor motor/trans/axles/everything you need for the swap for $1000.
It would take approx 2 yrs for the swap to pay out, from a fuel mileage perspective...and thats with the moderate numbers above.
That being said, if the goal is the cool factor, it'd definitely be cool.