Casting is 3894860nf and stamped code is BU0529G1(or i) and an E below the stamp. Also, where would I find a partial vin on the rear end? I know my engine is not original but I'm trying to validate my car being a 68 SS396 originally. Thanks for any help.

The casting number indicated a 12-bolt axle, as you have said. The BU0529G1 with the E below indicates a 3.73 posi axle, assembled on May 29. You'll need to look at the casting date of the axle to help determine the year. See the CRG website for details:
http://www.camaros.org/numbers.shtml#RearAxleNumbers

BU posi 3.73 12-bolt
0529G1: May, 29th manufactured at Chevrolet Gear & Axle on the 1st shift that day.
The E stands for Eaton, which made the posi carrier.

There is no partial VIN on rear ends. If the date shown above is shortly before the date your car was built, it'sa good bet that it's original. But there's no way of being positive without the build sheet/POP.

Looks like it is between a week and two weeks or so. My car was a June car so it it makes sense. Also, having all the original other stuff (steering wheel, big block heater core, special paint code and deluxe interior) I wouldn't hesitate much as showing it as a SS car without original motor and trans. Do you agree? It would be more of a stretch to say someone happened to find a rear end made two weeks before final assembly and installed it along with the other things I mentioned.