how to plug holes in ramshorn exhaust?

[hpcgmc]

I've got the original ramshorn exhaust manifolds from my 67 C10. I'm thinking of keeping these manifolds, since I hear they flow pretty well, without the leaking and burning plug wires that happen with aftermarket headers.

My manifolds have little holes from where the smog pump originally tied in with aluminum tubing. I've screwed bolts in to plug the holes, but now I'm thinking of having the manifolds powdercoated, so want a permanent solution that will look clean. If I just use steel allen head bolts before they are powdercoated, will they hold the powdercoating well?

Thanks!