smell

Driving while ago I smelled slight burn smell, check gages all good. Two minutes later, same smell gauges still are good. I down shift to fourth and I hear the rattling slightly, switch into fifth going down hill. More rattling and progressing. I pull over. There's a 5 inch split on top...

I have searched the forum, but there are 500 hits that come up dry as far was I was willing to look. My '72 Commando has the original 258, but the intake is newer, with blocked off emissions ports. The manifold heater is missing. I have a huge problem spitting raw and not fully burned product...