I can't think of any aditional castings for a 0-6-0 Sweet william apart from the extra pair of wheels. The only main difference is the extra length of the frames, can't remember if the standard boiler is used of it may be a bit longer (guess).
Everything else is just making more of what you need for the 0-4-0 – with some minor drawing changes.
May be worth getting a set of drawings for william and the 0-6-0 'pea' and measuring out the differences.
For instance the 0-4-2 'pea is exactly the same as the 0-4-0 but with an extra pony truck (helps stop the pitching).
I have a photographic build log for the 0-4-2 'pea if any use.
Regards
Lofty