Heatsink to dissipate 170W
Your heatsink volume is wrong. I am not sure how you got \$1801cm^3\$ specifically, but that range implies you just multiplied the external dimensions of the heatsink together but that heatsink is not a rectangular block of solid aluminum with those dimensions. If I do that, I get \$1.5" \times 6.20" \times 12" = 1828.8cm^3\$ Eyeballing the photos, the actual volume of aluminum in the heatsink is probably 1/5th of that.