if something draws 40 amps continuous, what is the start up amp draw ?
i believe the relay should be rated for the highest possible draw the item will see.
also, please do not use the Hobo Fright 1,853 piece terminal kit [only $9.99] to wire up these high draw accessories. quality terminals, as well as quality crimping tools are a must.
just my opinion and life experience repairing wiring nightmares. your mileage will vary.