The easy way is to look at the air cleaner mounts on the valve covers. On the older 450,500,560 engins, the rubber mount is screwed directly into the valve cover. On the 380,420 the mount is fastened to a bracket which is then bolted to the valve cover.
The reason for the extra bracket on the 380/420 is the 450/500/560 is about 2 inches wider.
84 500 SEL (307,xxx miles)