I’d guess a gasket equal to the blade thickness (and whatever was taken off for clean up on the mill).
I was thinking more along the lines of whether there needed to be a fixture to make sure the ports line up and intake manifold faces & the cylinder heads are still square. That way you'd be maintaining the previous geometry, but the welding would also need to act as a filler between the two haves, not just a welded joint.
Regardless, I'd expect a good bit of grinding to finish off the "reassembly" process, too.
