67cuda 360 oil pan issues

You need an oil pan from/for a passenger car 360.
273, 318 and 340 have the same seal front & rear. 360's only have the smaller seal in the back, as you discovered the hard way.

I'm not sure if all Magnum pans (318 & 360) will fit or only those for a 360.
*****Scratch that, because Magnums were never installed in cars. Only trucks & vans. People putting Magnum engines in a car have the same problem as you. But there are some Stock class racers that use a 2000(?) Dakota pan on their cars. It is rear sump, but may require some "adjustments" to the engine location.*****