A Weaind X-Celerator would not be a bad choice on you're motor. I have a Victor with a 950HP on my 408, fully ported Edelbrocks, 10.9- 1 with amost 20* duration at .050 though. It's gone 6.74 (1/8) spinning. In a 3300# Dart.

I think you're cam will be done at 6400 in a 400" motor. My 340 had a Hughes 3742 hydraulic, it liked to be shifted at 6200, it ran no quicker shifting any higher.