I'd use the version that currently exists which is a retconned Silver Age version that does not currently feature Mon-El (he's still in the Phantom Zone after the events of War of the Supermen). Back then though it would have been the Waid/Threeboot version I think.
Oh and I totally agree with Jetfire's comment regarding Supergirl. Instead of halfheartedly explaining that she zoomed off looking for Kandor based on rumors (where exactly did those rumors originate anyway and how would she have come by them? J'onn?) I would have written Kara showing Clark a Legion ring of her own and explaining that off-screen she had met one of the Legionaires who offered her a chance to come to the future and she tells Clark that is what she is going to do. "I have never really fit in here Kal, you know that better than anyone. There in the future, I have a chance to do something with my life, make a difference. I'll be back, but I don't know when."