I have a 81 280 zx with auto trans.

i am rebuilding for street racer.

can i get faster et with a higher rear end ratio?

Yes, the higher the number, say 4.11 instead of 3.54 will make for faster 1/4 mile and 0-60 times, but lower top end speed. Don't know if you plan to, but swapping to a manual trans will help out too.