[jdorour]

Hi Dan and Dave,
The headers from the 400 should fit the small block. The bolt pattern is the same for the big block and small block. The port size is smaller on the small block head. If you install big block manifolds or headers on a small block use bigblock gaskets. If you use small block gaskets with the big block headers the gasket will be exposed on one side of the port and will burn. The BBO is two inches wider and about 1 inche taller than the SBO. The BBO header will fit.
I have to add that I have never had good luck with headers on Oldsmobile engines. The bolt pattern does not apply an even clamping force on the gasket and you will fight leaks all the time. If you get a set of headers make sure the flange is nice and thick. The flanges need to be dead flat.