Based on the book SPECIAL TUNING THE MGB there is an extra 3-4hp by tuning the engine to stage 1.
1. Lightly grind and polish inlet and exhaust cylinder head ports.
2. Lightly grind and polish the combustion chamber.
3. Match up the exhaust and inlet manifolds to the cylinder head removing any ridges.
4. Distributor setting as standard.
Seems a reasonable diy job if the cylinder head has to come off for any reason.
More detail given in the book.