Yeah there aint no "manifold bolts".... all of em oughta be dowel studs'n nuts, self-locking castle nuts. And the washers should be one thick smooth faced washer with beveled edges on earlier versions.

Gotta order a fresh set of dowels, washers and nuts - thats what you've got to do. Now here's the news - good and bad - which do you want first?

On the darkside your 250/8 has been previously scotched by sloppy mechanics. On the brightside it has seen mandatory once-in-lifetime cyl head work done at/near 130k miles. Possibly prior repair of blown head gasket? Otherwise why should anybody pull the manifolds? It strikes me your manifold gasket leak is directly related to cheap labor attaching em. But lets hope they didnt strip fragile alloy cyl head threads in the process.