How hopped up is your 318? A 7.25 with a sure-grip is a surprisingly stout piece as I ran one at the strip for over a year on slicks. The s-g units are not easy to get though.
I will warn you quickly about 8.75 rears though as it is to easy to tie up $1200-$1500 in one without trying. It's not a bad idea to use one if you
plan on adding more power down the road. The s-g units new are anywhere from $600-$800.
For a 318 car like you have, I'd look for an 8.25 and find a sure-grip; The rears are not that expensive and the s-g units are not terribly overpriced either. New sure-grip units for these are in the $300 range. I've seen them as cheap as $50 in used condition though.
Hope this helps
-M