The 450 should have mechanical and vacuum advance in the dist. At 3000 rpm, the timming should be maybe 30 to 40 degrees.
To test, set base timming to 0 deg. Remove any vacuum lines from the dist. Then increase rpm to about 2000 rpm. The timming should increase 10 deg or so. I don't know exactly how much because I don't have the specs on that engine. If it doesn't increase the mechenical advance in the dist is frozen or rusted.
Next reconnect the vacuum line or lines to the dist and again increase rpm to 2000 rpm or so. The timming should now be 10 to 20 deg higer than it was on the first test. In other words now you should have mech and vac advances together. If test 2 failed then the vacuum advance is bad or the vacuum hoses are bad or miss-routed.
I think you will find both bad. Thats about the only thing that can cause the exhaust to get that hot.
Look into this and let us know what you find.
84 500 SEL (307,xxx miles)