I don't think its proper advice to recommend mounting an aluminum seat with a 5 point harness the same way you would mount a regular kart seat.
Though, the original post doesn't define which type of cage kart he has
Outlaw kart uses a front mount that is essentially an angle aluminum mount that mounts to the kart and can pivot slightly.. on the rear, there is a seat mounting hoop that bolts to the chassis and the seat bolts to it
QRC should show their stuff.. it will adapt to most chassis.
RFC has a good setup too..
Look up Blackhawk Chassis on facebook.. he will build what you need, and it will be good