Need help on 1969 L78 Nova posi

[68-69-70Nova SSL78]

I'm restoring a 69 L78 Nova (10C) build date. My posi rear end reads BU1230G1 which looks like it's comes past the car build date (won't work). I have the broadcast sheet but can't really find any documentation on what the original rear end was. OK so can I use any letter configuration lets say BT, BU BV, etc. for the rear end but dated before the car was built? I'm worried there is some code on the build sheet that will show a specific rear end. My friend says to use a BT rear end which would be more common 3:55 for that year. Also in the broadcast sheet 2nd row #4 reads CRD would anybody know what code would mean? Hope this is not to confusing. Any help would be appreciated. Thanks Ed