You don't need to fiddle around with equations. The dissipation is shown on graphs on page 14. With a plus and minus 28V supply and a 4 ohm speaker the max heating is about 48W.
The thermal resistance of the package is 1 degree C per W and the mounting with thermal compound is 0.2 so the total thermal resistance is 1.2 degrees C per W. The max chip temperature is 150 degrees C but select 130 degrees to be safe and select a max ambient of 30 degrees C then select a heatsink that has a suitable thermal resistance producing a temperature rise of 100 degrees C or less with a power of 48W.