You'll be ok. I used to use 5 gallon carboys and a blowoff tube. You will probably loose a bit-o-goodness, but it is better than losing 5 gallons.
You might also consider making a Buron Union system per Brew Your Own magazine. I have made one (but not used it) for those times when my 3 buckets and 6.5g carboy are in use and a 5g carboy is the only fermenter I have.
MATT - 65g in 2k8
3rd Circle Brewery
Ferminatin' - Route 666 Pale Ale
Figurin' to do - Oatmeal Stout, Gueuze, Belgian Blonde, Mild, English Pale Ale, Weizenbock