Are you certain that your RR oil pan flange is not dimpled or otherwise distorted? I've this same pan and found that torquing to recommended 15 pounds will grossly distort this flimsy/poorly designed flange.

I've posted before photos of my own little fix. If pan flange is damaged you will first require surgery to repair,,,hopefully successful. Once damaged and repaired, the metal has been compromised and will distort easier.

Used 1 inch wide by 1/8 inch thick steel stock, available at any hardware store.

Resolved my issues and all is well.

A major side benefit is with proper torquing, I am not required to use any silicone goop,,,I do use a bit at rear main seal retainer for insurance,,,makes for a much simpler oil pan RR next time.

I am aware that someone supplies a gasket with steel inserts or some such design at the bolt holes to help eliminate this distortion. I know available for Chevy(in fact I think factory built uses such), have not checked for Mopar.