360 oil pan/gasket

I have a rebuilt '77 360 that I purchased and came with what appears to be the wrong oil pan. There was not a gasket on the pan when I got it.

I purchased a gasket for a '77 but it does not mate up with the pan. The rear of the pan does not have the ribs like the the gasket.

From what I have found, it points to a Magnum oil pan and I don't see that a gasket exits to fit a Magnum oil pan to an LA engine.

So, I am seeking direction this. Is there a gasket that will solve this or am I forced to get a different pan? The pan needs to be a rear sump, it's going in a truck. If anyone has specific part numbers to note, that would be fantastic.