I use the cast summit unit on my 904. Its a nice piece, made in USA, and has thick end rails, which is said to help make the trans case more rigid. also, no clearance problems, it does not hang low like some people say.
I run a deep cast pan on my dart and it does seal up a lot better then the old steel type. The thicker flange doesn't bend like the steel ones do. I also run a gasket off of a early 2002 dodge diesel automatic transmission. It is the same as a 727 but it has a rubber o ring molded into the plastic gasket and seal up a lot better then a regular rubber or cork gasket and they are reuseable.
The late model pan and gasket from the dealer is what I use. The pan is not alum,just giving you an alternative. Put this pan/gasket on my new trans, and have 0 leaks.
The pan holds 2 more quarts,and the reusable gasket is the last one you will buy.
If you decide to go aluminum, use this gasket from the dealer.
Also check EBay for either or both...