I would normally be completely against such a swap considering the limited horsepower already available from a 616.

Very simply:

How do you consider the acceleration of the vehicle currently? Is it acceptable to you? Is it marginal? Would you be OK if the vehicle was 15% slower in acceleration?

Personally, I cannot imagine a 240 at 15% slower than it already is, however, if it will work for you, then you could consider a 3.07 axle from a 617 powered vehicle.

You would achieve somewhat better fuel economy from running the engine slower, however, I would be very surprised if it was more than 7% on the highway. There will be no improvement in fuel economy in the city.