Billet rear main cap... Double O-ring? Preference?

What is the Standard non-leaker. I'm a fan of O-rings, Is the Mancini Billet with them the way to go ? Also thinking of using the Viton Rear Seal too. is it worth it ? good combo ? Just not wanting a leaker after sitting all winter. Building this so It stay's together. Done and on to other projects and no redo's. beating after cruse after beating. 512", built 727 and looking for 3.55 gear for 8.75 or Dana also. Thanks, C300