Hi,
trying to pick a lens for birding and motorsport is going to be a compromise..

For birding (in this price range) I would suggest the 400 F5.6L, as per Marion's comment above and arguably the best BIF lens around. Will take a 1.4x quite well (but lose AF on the 40D). Not great for motorsport, doesn't have the versatility of a zoom and 5.6 will struggle to blur backgrounds on track etc.

For motorsport I would suggest the 70-200 2.8LIS - sharp, fast & versatile. Will also take 1.4x well. Generally too short for birding, even with 1.4x, and the 2x will reduce IQ too much particularly on small subjects such as birds.

If you must, best compromise IMO would be the 300 f4LIS + 1.4x. More practical for the track at 300 and almost as sharp at 420.

Cheers
John