Seems to me I rendered as 16:9 with letterbox activated and went to propertiers and selected reduce deinjterlace flicker to eliminate the motion arifacts. Couiple of weeks ago I produced a 20 minute show that was native 16:9 with a lot pf archival 4:3 B-Roll from BetaSP, U-Matic and Hi8. It was displayed on a 15 foot screen adn looked fine. To get the 4:3 the right size I went to the pan crop. Don't have much experience making a complete 4:3 to letterboxed 16:9, usually go in the opposite direction.
If it wasn't shot in 16:9, it's fake regardless. Shooting 4:3 with an anamorphic lens, shooting with a 'real' 16:9 camera are the only methods of achieving real widescreen.
That said, you can always create 2 tracks of generated media to create the black bars above and below 4:3 media to create the same look as pan/crop offers, but with greater control over where the bars reside. This is also useful for going over large numbers of tracks and you want to be sure everything is in correct placement. It will mask everything if placed on top.
BTW, you'll always be best shooting real 16:9 or using an anamorphic lens that 'squeezes' the image to a 4;3 screen vs having your non-native widescreen camera shoot this way for you.