I'm not a Buick expert but here's what Hemmings has to say about 12 bolt Chevy axles in Buick Stage cars in 1970.
https://www.hemmings.com/magazine/1970-Buick-GS-455-Stage-1
What to watch for: GS 455 Stage 1s were automatically equipped with reinforced upper control arms that were not available on the base GS 455. Furthermore, Canadian-built GS 455 and GS 455 Stage 1 cars were furnished with Chevy's 12-bolt differential; however, it featured Buick casting numbers rather than Chevrolet's. A Space-Saver spare became available for the first time.