Sorry about the late reply, I was on a holiday making scanner fodder :).
The problem is that there is a restriction in the hardware which limits the width of of a scanned image to about 38 mm. The trick is to combine two images. It can be done in Vuescan (don't know about Canoscan) because Vuescan allows you to offset an image. It is slow work but goes as follows:
- Use the film holder, not the holder for the framed slides
- set Device|Options types|Advanced so you have full control
- set Device|Media type|Image
- set Crop|Crop units|mm
- uncheck Crop|Crop auto position
- preferably save raw ('scan') files, though it seems to work with 'crop' files as well, but then you maybe will get color differences
- do the first preview, adjust cropping and colors
- lock exposure, do a second preview
- lock image color
- do a scan of the first part of the slide and save it
- set Device|Frame offset| to plus or minus 9.437 (I have to admit I don't really understand this number, I found out by experimenting; the plus or minus depends on whether your image is aligned at the right or the left side at onde of the vertical bars in the holder
- do a scan of the second part of the slide and save it
- combine them in photoshop

At the moment I do the combining with the photomerge function of PS Elements, but this limits me to 24 nit images. If someone knows of a programme which can merge 48 bit images, I would be very interested