[BLSTIC]

Or you could tune the engine *exclusively* for low-rpm and run a relatively large turbo (or, you know, two).

A large turbo has a number of advantages in your application

1: Lower backpressure - Reduces pumping losses and likelihood of detonation. This means you can run higher compression than you otherwise thought and your engine makes more horsepower per psi. Also uses less fuel on the highway.
2: More efficient compressor for higher airflow - more power per psi thanks to less power draw from the turbine
3: Higher boost threshold - not *usually* a plus, but in your case you already have a disgustingly large amount of torque available at low rpm, it would be difficult to do high load efficient driving if you had 500ft-lb at 2000rpm
4: You get to run a smaller cam and high compression. Because the boost won't be there at low rpm, you can have high compression. Because the cam is so tiny at high-rpm, the boost is merely bringing things up to speed again and your compression ratio isn't too high for the actual amount of air in the cylinder.

On the one hand, your turbo V8 only makes 400-450hp. On the other hand, you have a turbo V8...

The really good news is you can do this *right now*. Rear mount turbos are a thing, and moving any turbo to the back of the car increases its effective turbine size, a chinese $150 T3/T4 with an internal gate would be ideal for an experiment. It also means that boost response is exhaust temperature dependent so you may end up with a car that you can't accidentally boost when it's cold. And has epic spool noise