Troy with a Hemi Dart and Barracuda now runs those 3 row high density cores the OP has. Last one from Glen Ray

He has viscous clutch with OE fan. No shroud cause factory hurst S/S didn’t come with them.

No cooling issues with a 528 Siamese bore and 472 regular bore motors. 90 degree So Cal valley weather here surrounded by mountain ranges on to get out and in.


With the items the OP has done so far, I think there needs to be a look at non cooling system issues:

Ignition
Carburation
rust and scale in block
Physical failure: head gasket, head crack, etc
Mechanical: cam lobe flat,

383 cars were often raced and lived hard lives. Do you have any idea the condition of the block?: O-ringed, sleeved, etc. numbers matching?

OE Hemi cars with original/dated motors can have nagging problems like this cause the Hemi parts used to build them are all raced-out. Repaired blocks are typical, large overbores, all kinds of crazy stuff done to them.