'72 Duster taillight ?

Evening all,
What is the secret to installing the rear taillight assembly?
I have new gaskets and do not see how to get a good seal with the body.
FSM simply says to slap it together, actually that's my interpretation, the FSM reversed the Duster and Valiant and says the bumper has to be removed!

Should I glue the gasket to the tail light first to hold it in place? Does the gasket seal against the lip on the tail panel, or does it seal against the tail panel? This is driving me batty!

Unfortunately, my tail panel was so rotted out that I didn't have much of a guide when I disassembled. I've replaced the tail panel and "think" it was a good piece...
Thanks,
C