906 or 915 ?

The OP says stock looking with under chassis headers. This isn't a concourse restoration. So it seems there is some leeway in what happens to this GTS. Stock looking would even allow a stroker kit.

One thing to remember if 'stock looking' includes a year correct stock factory cast iron intake manifold, that manifold will limit airflow to around 200 cfm even if your heads flow 320 cfm. Yes, I've personally made that test.

450 horsepower isn't mild in my opinion, that is more like ripping. This thing is going to be more than stock to go 450 HP, or a stock HP 440 wouldn't be only 375 HP. 450 is going to be dangerous in a Dart.

Ah, what the heck do I know.