definitely lift the ground bus away from the heat sink (it only has to be slightly away). Even with a real big soldering gun, if that copper bus is tight against the heat sink, it will be virtually impossible to solder.
I service a lot of commercial Class E stuff (13.56Mhz 3KWRF out). One trick I use is to warm up the heatsink with hot air before and during soldering the power components in or off the PCB. I have a heat gun suspended overhead, adjustable height from 6" to 18" over the item to be repaired. Sometimes if the amplifier works, but is sick, I let it run without water cooling till the thermal OT sensor kicks in. That brings the temperature up to about 65 degrees Celsius. Then quickly move it over to the rework table to pull the power devices. Using heat allows a smaller soldering iron to be used, and reduces the thermal stress to the semiconductors.
Jim
Wd5JKO