out of curiosity what kind of number are you trying to make? I am also in the process of building a twin 88 BBF combo and am yet to identify what heads to run. Do you think the p-51 provide enough flow to make the big number?
Twin 88s are typically good for anywhere from 2,500 to 2,800 hp. Not sure I ever want to max the turbos out, but I'd like to run 7s on a low-boost (15 psi), pump gas tune.
IMHO, I think the P51s are very much up to the task. From what people much smarter than I have told me, a max effort P51 head will flow in the 430 cfm range vs. 490 cfm for a max effort A460 head. On a naturally aspirated motor, you'd be crazy to sacrifice those 60 cfm (granted you can afford them), as they'd be worth an extra 120-150 hp on a high compression race motor. However, that same logic doesn't necessarily apply to a forced induction combo.
Boost and airflow are two very different things. In essence, boost is merely a measure of restriction inside the intake manifold. Let's say a 532ci combo with twin 88s and A460 heads makes 2,500 hp at "X" boost pressure. If you took that same motor and put P51 heads on it, the motor would make the exact same hp but at a slightly higher boost pressure. Sure the P51s would be more restrictive, but that restriction isn't nearly enough to reduce the total airflow capability of twin 88mm turbos.
An extreme example would be the Titan Motorsports ADRL Scion. With a single 91mm turbo on a 190ci inline-six, it ran 4.30s in the 1/8th at 50 psi of boost. There's no way six intake ports on a Supra head flow nearly as much as eight P51 intake ports, but by cranking up the boost that little six-banger makes enough power (with no weight break) to run with the twin-turbo big-blocks. That same 91mm turbo would make less than half as much boost on the big-block as it does on the Supra motor, but not any more power. Actually, it might make less power since you'd have to spin the snot out of it, take it out of it's ideal efficiency range, and seriously heat up the intake charge.
Since then, that car has run 3.99 in the 1/8th, but I'm not sure how big the turbo is or how much boost it's running.