You can't use those 4.30s in a Miata nose housing. The pinion gear is too long.

 

I don't think it matters at this point since he'll be making a custom driveshaft. Might as well keep that rx7 diff intact. Unless I'm missing something.

You'll likely have to rebuild the clutch type diff though. I bet the clutch packs are toast.

 

i guess im just gonna leave the long nose on. its hard as hell to spin the axles in opposite directions. i wanted to make sure it wasnt welded.

 

Actually, I kind of like the idea of using the long nose rear end. It will give you a much more convenient place to bolt a diff mount to, instead of having to build something around the PPF mount on the Miata rear end. Plus, you can use more readily available FC gearing options.