I am guessing sending this out to a dub house for a high quality upconversion is not in your budget…and also you do not have a Kona or Decklink, which can do a decent upconvert on the fly during capture…
If you want to work without rendering in FCP, you should probably do this in 2 stages.
Export from Streamclip as Uncompressed 8 bit SD. Then take those files into Compressor and upconvert them to DVCPRO HD 720p/59.94. In the geometry controls in Compressor, you can select to pillarbox the 4:3 image in a 16:9 frame or blowup/crop to fill the frame. Be sure to turn on the Frame Controls and set the Resize to Better or Best, as well as the de-interlacing.