Are your heads ported or milled? I imagine through a locked converter you would definitely see over 400 whp. I'm running 11.8:1 compression on mine and looking to be over 425 whp through a 6 speed with matching torque.

 

Heads are mildly hand touched up on the intake side and intake/exhaust ports had the casting flash knocked out. Milled .010" for clean up.

That dyno day was kind of impromptu, had drag radials on at 20psi and cutouts closed (at the track cutouts are worth some ET and MPH). Someone else paid for me to do a pull and the only reason I did was to see the top of the HP curve and make sure my wideband was accurate with theirs.