I wonder if it would be unwise to follow this approach? Its apparently more common in germany when they do the conversion.

My main concern would be the filler that's used? What would be a permanent solution? And what would give the best seal?

If I could get a good permanent filler that would give a good seal I would get a new M52 manifold plate (rather than reuse my one) and take this approach. Lot of "ifs" though.