In all my years around 40 series I have never seen any type of gasket used between the step and rocket panel. Think if I did anything it would be to use clear tape similar what Toyota started using on the front edge the hood and now other areas like the bottom of the rockers to protect from small stones being thrown up. But I would probably just be sure both surfaces are smooth and apply the paint extra heavy where to two meet. Same with the front edge of step against the fender. Using a thick gasket would use longer bolts and use a second nut to prevent the bolts from working loose with the flex between the two which Toyota did not intend to be there.