I would advise you to check out very carefully, the way those 273 exhaust manifolds lie on the 360 exhaust flanges of those heads for this reason: The ports in those manifolds MAY (and I am not sure at all about this, but it's possible) not totally cover the exhaust ports in those heads. The 360 exhaust ports are much bigger than the 273 ports.
Maybe you need to get a 360 gasket and lay it on your 273 manifold and see how the port openings in the manifolds look as far as matching the gasket.
Also, and this is important, some smogger 360 heads have a hole drilled in the face of the exhaust flange below the bottom edge of the exhaust port, on the head. It's an air passage for the A.I.R. pump hardware and it's functional on every cylinder.
These will have to be plugged, but that's easy. Forget to do it (if your heads have them,) and you'll have 8 smallish exhaust leaks that won't make you too proud of your exhaust system. LOL!