I have a new Dayco Top Cog belt, part number 15625, which cross references to GM 1378322. According to my By the Numbers book by David walker, this belt is supposed to fit 1970 GS 350 with a performance axle. I believe it fits many other Buick applications as well. Not sure why I have this new belt, but I found it in the bottom of my toolbox and has been there for a few years. I am asking $10.00 plus actual postage from Ottawa, Ontario.
