400-425rwhp feasable?

Assuming:
1. A well-balanced assembly to reduce vibrations/harmonics
2. You don't get too aggressive with your cylinder bore
3. It is designed for relatively low RPM duty
4. A great breathing head-intake system so that you can reduce the need for excessive blower PSI
5. You can effectively control engine heat through conservative tuning and an upgraded cooling system
6. Thick-decked heads and engine girdles help reduce block expansion and flex that often create blown head gaskets and/or engine failure

If you go onto some of the turbo Mustang and speed shop sites, there are a number of TC/SC 'stangs with stock blocked 306's running in excess of 600 rwhp. In addition, several NMRA Pure Street cars are running 400+RWHP naturally aspirated. So, 450RWHP is a realistic number for a street/strip blown combo.