Isuzu rear ends are going to be a good bit larger than 6 3/4" Ford... you're talking about 8.5" for an Isuzu 10-bolt diff. You're not going to find a rear end smaller than 7.5"-8" under a truck/SUV. for something like that, you might wanna look to car axles.

Or, like steve said, built it overkill and go with an Isuzu 10-bolt or Ford 9" if you want to go totally crazy.

-Rob