hi brent,
I did a timelapse last year (first half mostly cloud - and not properly aligned)
https://www.flickr.com/photos/80336656@N07/15291875979/
I would recommend taking more photos - I did one every 6 seconds I think from memory.
I don't know if you can on the 450d but I was plugged into BYEOS but did not use it - I used the magic lantern intervalometer. the photos still are downloaded to BYEOS so you can see clearly how the images turned out.
the other advantage (and important in a dynamic event like an eclipse) is that you can tweak the exposure in between photos - simply by pressing iso or swivelling the exposure (which you can't do on BYEOS) without it interfering with the set intervalometer. I changed it constantly on mine to get a result through the clouds.
cheers
rusty