I don't know what other people use (besides cast iron welding) but I would use water pipe plugs. But you need a pipe thread tap. Pipe plugs are made of cast iron so the expansion rate will be the same as the cast manifold. Treblig
Exactly. The smartest and easiest thing to do and it will last forever. Just find the next size NPT plug and tap it to that size, put some high temp RTV on the threads and plug 'er up.