What's an 8 3/4 A-Body Rear End Worth

$800 is a little high, but if you're staying with the SBP axles, and all the hardware, drums, shock plates, and U-Bolts are there, it's not too terrible.

You could buy a 3.23 open diff for around $150, maybe less depending on the carrier.
SS Springs are $240/Pr from Summit.

- my 2 cents.