Pan gasket for 360. What one works best?

So the part # above is a mopar part ? from a dealer. I'm using a stock pan, but a magnum gasket, using the ends from a felpro gasket set. I wonder how the rear main seals, mopar and felpro are different. So in your case the rear main seal was the problem.
Is changing the rear seal a big job?, whats involved?
Maybe thats Ed Beight's problem also
Thanks for the help