Sasha’s method is best as you can individually re-frame each shot
but you don’t have to use the widescreen mask filter,
another approach would be to drop your finished film into a new timeline: same settings for everything but set to anamorphic.
oh, yes, and go into the sequence settings, Video Processing, and turn motion scaling to BEST
it wil most likely appear with black bars left & right (Pillar-boxing)
you need to BLOW UP your footage to 133.33%
just like Sasha’s method you can control each shot, adjusting the framing up or down, key framing it if needed,
even leaving some shots stretched wide if they work best that way.
(i have even dome some simple composting to make 4/6 shots work as 16/9)
main difference is you now have a 16/9 anamorphic version of your film,
not a 4/3 letter-boxed one.
which you chose depends on your delivery requirements.
oh, and back to the widescreen mask, it is MUCH easier to add a mask as an overlay on a track above your video , and reframe it underneath the mask
nick