I am sure few people swap V8 for V8 in newer mustangs. The V6 was discontinued for Mustang in 2014. Ford made such notable progress in developing modern technology for its new V8's that they really set a new benchmark for Chevrolet and Chrysler. That is one of the things I admire most about Ford, their willingness to innovate and take risks that other car companies don't. And they succeed.
I am not swapping a V6 for a V8 and I don't know why that was the assumption. I will be swapping the V6 into a car that presently has a turbo'd 4 cylinder. The 3.7 weighs just 100 pounds more than stock and has 305hp, normally aspirated. The present engine has a whopping 152-200hp even with the turbo. The 3.7 is light and narrow due to the 60 degree configuration and can easily be modded with a turbo to yield aver 400hp. Some guys have taken it up to 600hp.
1. Nobody made the assumption you wanted a V8. Go re-read the previous posts.
2. Seriously, the logistical nightmares that come with powertrain swapping modern cars, especially Fords, mean that there is virtually zero swapping being done these days outside of race cars. Even then, a 3.7 car is a worse foundation than the 5.0 cars in several ways.
3. I know what a 3.7 is capable of. A co-worker had a Roush 3.7 Mustang with a blower, even with an autotragic and a relatively conservative tune it was at 420rwhp. It's a great engine in it's RWD form, if woefully mis-tuned in the F150, and a time bomb in it's FWD form.
4. Ford didn't innovate a damned thing with their newest V8s. The Godzilla is a step backward (although GLORIOUS in its execution), and everything about the Coyote and Voodoo had been done before by the Asians and/or Europeans.
5. The 3.7L wasn't discontinued in Mustangs until after the 2017 model year.
If you want a 3.7, check with salvage yards, or see if you can snag a whole wrecked car on Copart's website for cheap. The odds of finding the one unlikely person crazy enough to tackle a 3.7-to-V8 swap to buy their old powertrain from is virtually zero.