Small block Chevys use a 168 or older ones use a 153 tooth ring gear. Be careful what 350 you may be shopping for. When shopping on ebay and you search for Buick 350/whatever, you will get sbc parts marked "will fit Buick" when they are for a sbc only motor. This is because the sbc 350 was put in the Buick after the Buick 350 production was stopped. This can probably happen with other searches too.
Here are a couple on ebay that should fit.
We try to list the years and models as best we can with the information we have for interchange between models.
rover.ebay.com
Find many great new & used options and get the best deals for 1966 67 68 69 70 71 72 73 74 Buick Skylark GS Special LeSabre 340 350 Flex Plate at the best online prices at eBay! Free shipping for many products!
rover.ebay.com