[c-tone]

In tough cases such as these where time-stretching is almost always audible to some extent, I usually end up not time stretching the exposed long sustained notes. Instead, I'll spend a lot of time experimenting with duplicating the sustained notes to other tracks, offsetting them, and doing a combo of crossfading and fader rides to make the notes longer without the artifacts.

I also sometimes use regenerating delays to extend the long notes, but in this case with so many stereo pairs, it probably wouldn't work so well, but it's worth a shot.

It will take a lot longer to get right than stretching, but it can be more transparent.