Safe RPM for a 408" Magnum?

if I'm not mistaken, your limiting factor is the valve train

it might make more sense to get the transmission to shift earlier,

Normally the valve train is except the cam isn't supposed to be turning that high of a rpm and the real limiting factor here is the stock oiling paths.
Shifting the trans sooner is the answer. If it doesn't do it by itself then you need to look into why and fix it/adjust it.