all you can get for that would be an old sigma 400mm f5.6. doubtful it would work on digital due to newer electrics but if you look out for a re-chipped one on ebay then thats your best bet. dont buy one if it hasnt been chipped as it wont work and sigma probably wont be able to chip it anymore because parts wont be available.
other than that option, a 70-300 is your longest choice.