If the manifolds are the originals, they are a one year only manifold which has a little larger out. The manifolds are a little more delicate than the '68 and up and it is common to find them cracked. Don't know what your rust level is, but usually if you blast them and put some good manifold paint on them, they can look presentable.
I don't know if there are new replacements available, but any big block manifolds will fit. You should be able to find a set on ebay for a couple of hundred dollars. Click Here for Buick exhaust manifold
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