It wasn't not the overlaid tiles that needed padding, it was the tile beneath (3240 and 3245)
The reason? a 428x200 tile (once stretched 20% vertically) doesn't actually have an exact 16:9 aspect ratio, it's just slightly wider. Nothing you would notice by itself, but once you start adding other tiles on top of it, it becomes noticeable.
By padding tiles 3240 and 3245 to 1926 pixels wide instead of 1920, it has the same ratio as the tile it replaces, and tiles 3241-3244 and 3246-3249 will align correctly.
I'm attaching the padded version of 3240 as an example. It was saved with Paint.net so you probably want to use PNGOUT to get better compression