What frame rates are in use here?
Is your widescreen footage sd or hd?
Looks like you are just going to have to make all the widescreen footage into 4:3. This is one of those "only as fast as your slowest man" situations.
Both lots of footage are 29.97 fps
If it were me, I'd probably work in a 4:3 sequence and crop the sides off the 16:9 stuff.
As I understand, there are 2 ways of converting from\ into
One situation is when I convert the 16:9 into 4:3 and I add black space in the upper and lower part of the footagage, and it is called letter-ing.
The other is 4:3 into 16:9 and here I add black to the sides and it is calle pile-ing (I'm not sure if spelled corectly).
In these 2 ways you will not bring deformation to your footage.
The black bars are "letter boxing" (top & bottom), and "pillar boxing"(right & left side), so you were very close.
I think that the OP is trying to get around either of those results, wishing to fill the Frame Size, 100%, or very closely.
In Jim's method, one would do the 4:3 Standard for that material, and then use the 16:9 Widescreen with the vertical resolution, but just effectively crop off one, or both sides (using the fixed Effect>Motion>Position). This will keep the quality as high, as is possible. When one takes 4:3 Standard to 16:9 Widescreen, one must use scaling, such as Motion>Scale, and this will degrade the image to a degree. Also, one then is faced with effectively cropping top, bottom or both.
Now, pillar boxing with the 4:3 material will certainly work, if the OP can live with that look.