Submerge it in mineral oil, and circulate it directly onto the chip faces that have an epoxy-heat-sink on top, and a through-hole to a sunk-ground on the bottom.
Beyond that, you can still cool them from the top, with epoxy-heat-sink that is either segregated and vacuum-bonded to the surface, or a standard thermal setup with "spring-tension", that will not damage the package with massive torque-pressure. Yet, still allow the heat-expanding aluminum and board to slide as needed.
The spring tensions can be simple push-through "V" tabs, attached to the heat-sinks directly, pushed through slots in the pcb, between the actual chips along the center-line of the board. (Not at the edges of the heat-sink... You want a center-line so the least movement is centralized and pressure around the whole surface uses the other chips as "supporting" feet to rest-on.)