360 oil leak at back of intake manifold

If all else fails, double check your valve covers.
The lip around many aftermarket valve covers actually comes in contact with the top of the intake manifold above the runners, preventing the valve covers from tightening down properly and rendering the gaskets ineffective.
The common solution is to make note of where the interference is and grind the valve cover lip at the offending locations. Yeah, it's tough to think about taking a grinder to those nice shiny new expensive valve covers- but sometimes ya gotta do what ya gotta do.