The swap doesn't make financial sense, no disputing that.
Same issue I went through when I swapped in the 518. The fuel savings doesn't justify the cost, unless you drive 20k miles per year, which most of these cars don't.

Same situation going from the tired 340 to the 5.7.
I picked up 150HP and 3-4mpg, could have rebuilt the 340 for ~5k less than the swap.

But in both cases, the 518 and 5.7 have both made the car so much more enjoyable to drive.
I plan on driving this car a lot going forward.
Taken several 4hr roundtrip drives that I would have never considered, even with a rebuilt 340.

So in my case, the new motor, upgraded tech and driveability was worth every penny.