Hi Eugene,
As far as I was aware the heads on Buicks are the same 32-34 at least in 50 series.
It might be bestto double check with Bob Engle as he knows all about the 32 50 series. He is well know on this site.
Thanks Jon
If the assembly is fine your best option would be to machine a new groove for the bolt to hold the whole lot back an extra 1/4" or check the tightness of the bolt holding the fan shaft.