Paul,
There is either something wrong with your 6.3 reducer, or you have the spacing incorrect.
As an example of what image size you should be able to get with your scope, check out
this pic taken with an 8" SCT with 6.3 reducer with an Apogee 16 megapixel CCD. Various sensor sizes are indicated, your camera would be the third size out (or third size in), and you should be getting very minimal vignetting with your 350D.