I have digitaliseert many 8mm movies and where possible made them 16:9.
you have to realize that in many cases you have to make a choice what part of the original movie you want to keep and show.
in many cases it is making a compromise. There are cases in which it is better to leave the original format as it is.

You can use crop and zoom and framing to make your pictures 16:9. What I usually do is cut the video into scenes. Scenes that more or less require the same cropping. For each of the scenes you apply (in tools) crop and zoom. Set the format to 16:9. Also do that for the whole project! And select the part of the scene that you want to show. You can in de zoom screen see the result in the top right hand corner, but that is a very small. So press OK and preview the cropped scene from the timeline, and open crop and zoom to correct.
In a number of cases you may want to move within the scene from one part of the picture to another part. In that case use the setting of frames within the scene. It may take some experimenting and some experience, but the result may be rewarding because it lookes like the camera is moving its focus at the time of shooting! PD will take care of the smooth movements between the set frames.

There are some good tutorials in YouTube that will help you!

Quote With 365 what is the best way to make 4.3 video into a 16:9 one ?

Know it could cut off top or bottom a bit, but how can I see to test it please?

You have several approaches, pick what works for you.

A) Keep 4:3 with sidebars.
B) PIP approach with the 4:3 file, a pic or other stretched to 16:9 and heavily blurred (like you often see when broadcasts use a cell phone clip).
C) 4:3 CLPV to stretch to 16:9, works better in some scenes than others. Does distort but keeps the center at a minimal distortion for proper perspective
D) Zoom and crop

Jeff