Mixing formats/Getting project to fill entire YouTube screen

Hi Everyone.

I have scoured this site and haven't found my particular my situation so here's my issue:

The source material I'm working with has five different pixel dimensions/aspect ratios: 1920 x 1080, 720 x 480, 720 x 486, 864 x 486, 1280 x 720.

What is the CORRECT way to assemble these on the timeline so that when I export and upload to Youtube it fills the whole screen?

Here's what I have tried that is NOT working:

I have tried just using 720 x 480 sequence settings since the majority of the footage is this size. I then put Andy's letterbox filter over the whole show so that it all looks 16:9. Then I physically moved the 720 x 480 footage within the letterbox filter so heads aren't chopped off, etc.

This seems to look fine within the timeline, but after uploading to YouTube there is black frame around the video. I researched YouTube and found a formula you can put in the tag box (yt:stretch=4:3 yt:crop=16:9 yt:quality=high) which resizes your video to fit the dimensions of the YouTube player. But then the video loses quality and looks ratty.

The new Youtube dimensions are 1280 x 720. So I also tried starting my project using those sequence settings but once I started cutting the various sized clips into the timeline, nothing looked sized right. To further complicate things, the graphics for the project were created using the HD templates in Motion which are 1280 x 720.

Can someone please tell me the proper steps to create the best quality output that will look uniform and fit YouTube dimensions.

Thank you in advance.

Shelley

Shelley
MacBoo Pro 2015
16 GB Ram
OS X 10.13
Premiere Pro CC