[TwiztidK]

You're dead on about rigid mounts for the pedals. One of the first issues many sim racers run into with regular cheap pedals and a desk mounted wheel is that they pedals slide once you start braking hard. And with load cell pedals the expectation is that you'll be using even more force, so that problem will just get worse.

As for pedal mounting location, many pedals can be top or bottom mounted if your racing chassis allows for it. However, floor mounted pedals offer the same performance as top mounted, match the majority of purpose built racing cars, and are simpler to mount in a sim racing chassis.