Don't dismiss car because of the 10 bolt...wouldn't be the first time somebody broke a rear and used another one that was handy...check the axle stamp code...69 model year will have a 2 letter code.....the POP will tell the story...the L48 was SS only. Good luck....by the way, you mentioned power brakes...but are they disc..??? don't rule out build sheet in the front seat...I have found several there over the years...
wilm
