I'm not sure if you can put two turbo's on a V8 and only make 500hp... lol
All kidding aside...I'd try two Mitsubishi 14B turbo's off the DSM stuff (Eclipse/Talon). Small enough that they'll spool, and they will support 500hp on a 318 with no issue. They are internally wastegated too, so it'll save you that expense.
Also why 7.25:1 compression? Are you trying to run non-intercooled 87 octane?
The low compression will hurt the spool up time as much/worse then the A/R will.
On my turbo car, everytime I dropped compression the spool got better and it made more power everywhere. I am in the process of dropping from 8.5 to 7.8. It seems counterintuitive but on a boosted car the lower the CR the better.
