Should I gasket seal on transmission pan?

Another avenue for you to consider, is to use the factory gasket that is used on the A500's and the A518'S. As I recall, its a rubber type material with a steel insert. GM & FORD when they first started using them called them a Zero Leak Gasket, and having owned a couple of Chebbies in the last 15 years, I can testify they don't leak. I used this type of gasket on my Challenger when I serviced the trans the last time, and NO LEAKS. Only problem is, you must torque them to factory specs with a torque wrench.
Also, as others have pointed out, make sure the pan rail is flat, especially around the bolt holes.