I agree.
My car was probably in the area of ~530 at the crank, on 93 octane gas and a stock bottom end car with cams and probably every bolt on. Other people have dyno’d 500+whp on pump gas, but as you said, it becomes expensive. I feel as though on my setup, I maximized my use of airflow pretty well. If I wanted any more on pump gas, I would’ve had to change some internals around. This was all put through a 5-speed though, and you’re going to see more drivetrain losses through that auto.
Is it doable? Absolutely. But as I said earlier, I hope he has deep pockets.
I think a more reasonable option would be :
-build the auto tranny
-buy the flavor of the week 50 trim
-get a 6-bolt block if you ever develop any problems with the current motor
-make around 350 on pump gas hopefully, and enjoy a low 12 second car.
Just my .02 though.
Jesse