I dont think anyone knows this answer... I have a 69 with a 383 all done up, 6 pack as well. I know what you are going through cuz it happens to me all the time. A new rad or even a proper rad flush...
GTX comes with a hemi standard. Roadrunner does not.Actually the standard engine for the GTX was the 440-4bbl, the 426 HEMI was an option, throughout it's entire production run as a stand-alone model...