There is no torque spec. The floor pan is a tuning item, so you can run it loose and floating or tight.
Similar to a torsion bar, a stiffer floor pan (or tighter) will make the kart quicker to react, both in lifting the inside rear wheel but also setting it down. A softer (or looser) floor pan will allow for more flex in the center of the kart, similar to a softer torsion bar or softer seat.
On some karts I prefer to swap out the floor pan hardware with longer bolts so I can run it floating without fear of the nuts backing off.