I have been running a procharger HU on my Corvette for 4 years now.. It was making 657 rwhp with 8psi. I have since built a 383 stroker and am going with a D1 procharger and will have around 12-14psi and should make north of 800 RWHP. Granted, it is on an LSx engine and not a hemi, but the concepts of supercharging are all the same.
You also need to think about other things.
Boost-a Pump, or larger fuel pump
Tuning..<-- this is KEY
You can run 3psi and if your tune is not spot on, you can blow an engine. Your timing, A/F, spark tables, knock sensors, O2's have to all be tuned and spot on. The procharger kits are proven and have been out for years.
Check this kit out and call these guys to discuss.