If it's designed to fit '64-'65 273 heads, the angle of the bolts that hold the manifold to the heads is different. If it was designed to fit '66 & later 273 heads it would bolt up but the ports on the manifold would be a lot smaller than the ports on the 360 heads. It might be possible to open the ports on the manifold to match the ports on the heads. It would just depend on how much material there was in the manifold.
If your only requirement was to bolt a 2 x 4 setup onto the 360 heads, it could be a straight bolt-on, provided it was the late 273 style manifold. The late 273 heads are pretty much the same as 318 heads, as far as manifold swapping, port size, etc.
