Ultimately the answer is going to be, "Try it and see if it works."

I can add that some of the 601 based machines did ship with a flat waffle iron type of heat sink similar to what you have in the picture there. The early Power 80, Power 100 and Power 120 machines from Power Computing used a flat heat sink rather than the cage type found in the 6100/7100/8100. Also, the heat sink on the PPC601/120 CPU daughter card in the early PowerCenter used the flat type. However the flat ones all covered a little more surface area than the square cage type heat sink.

With a fan, it will probably be okay.

Be sure to clean the old heat sink grease off of the CPU and apply new grease. Having fresh grease will go a long way towards improving your cooling.