When I bought my 89 Sunsport a couple years ago, the manifold plugs were pretty corroded and the riser gaskets had a small leak. I ended up replacing the gasket, which involved removing some very rusty unpleasant bolts, retapping the threads, and replacing the plugs with new plugs (http://www.skidim.com/PIPE-PLUG-3_4-...ctinfo/RS3533/). Given the angle of the exhaust coming off the riser, you really can only retap these threads with the riser off.
Right now I can remove the plugs with a socket wrench without trouble, but I'm not keen to see the threads deteriorate over time and need retapping.
I'm thinking about replacing the plug with a valve system. Skidim has a stopcock drain (http://www.skidim.com/EASY-DRAIN-EXH...ctinfo/MCM003/). The guys on the CC pages think this is likely to jam with a little corrosion and recommend using a larger bore drain cock like this: 6D912_AS01.jpg (https://www.grainger.com/product//AN...ain-Cock-6D912 ... yes it will need a 3/4" to 1/2" reducer).
Has anyone had any experience with a manifold drain? Just keep using the new plug and hope for another 20 years on it?