There are, as you suspect, two FMX pan gaskets.  The light duty one is basically rectangular, just a slight offset at the left front.  The medium duty has a bump out in the pan on the right side, just behind center.  Your parts store should have pictures in their book to identify it.  The shifter has no bushing, only a seal, and there is an O ring where the kickdown shaft goes through the shifter shaft.  You have to take a couple nuts off from the inside to get the levers out.  Valve body has to be out to do this.  Then you'll have an aggravating time getting the spring loaded detent back in place when you are ready to re-assemble.