Workflow for an edited 5K sequence with a 2k or 1080 finish

HI,

I've finished editing a project in Premiere.

I edited the film in a 5K sequence (5120 x 2160) with an aspect ratio of 2:40.
Yes, I could of edited the film in a timeline with the intended delivery format and
perhaps saved some headache but I'm trying to figure out the most pain free way
to take my edited project and move it to a 2k or 1080p timeline with 2:40 aspect ratio.

The reason I need to do this is some of shots have been scaled upward and I need a lower res
timeline, so it doesn't look terrible. Most of the shots have no scaling applied.

I've tried just copying and pasting and checking the scale to frame size into a 2k or 1080p timeline and
it doesn't replicate the footage correctly.

Looking for help or the exact frame resolution needed to make 5120 x 2160 (2:40) look correct in a
2k/1080p timeline (either works for me whatever will be the least hassle). Ideally I'd like to be able to next the sequence and hit scale to frame size, and all the shots would look correct. Except for the shots I scaled, I'll have to re create them in the correct timeline resolution.

I need to clarify one thing, I worked in the following sequence. When I put a clip into the timeline, I kept the sequence settings.



This delivered the appropriate 2:40 image with out seeing the extra footage outside the frame and when I exported I got the correct amount of Black space for the letterbox.

In a 2k or 1080 sequence, the closest I can get is by placing the clip in the timeline and scaling down 40%. It's close but still a bit off from the image I see of the same clip in the 5K timeline.

Unfortunately, the whole "scale to frame size", won't work for me in this instance as it pillerboxes the sides,
I'll have to scale all clips.

Is there a better way to go about this? For instance take my edited sequence into a native 5k timeline that
matches the clips specs 5120 x 2560 (1.0). Then move it to a 1080p sequence, scale to frame size, then
crop to 2:40 upon export?