I can't really advise on MPEG Streamclip with respect to what you want to do. But in general, if you want to enlarge to more pixels, you may have to blow up (or stretch) the pixels. This may cause some blurring. Having said that, you may want to test this out by using the 1280 clips and strecting them in FCE in a 1920 sequence and then have a look how it looks like. Depending on the shots, this method could do the job.