have seen some posts suggesting that you could adapt a larger system to a smaller flue, but couldn't find anything definitive.
Here is a definitive answer backed up by experimentation. If you have an 8" system, you can't use a 6" stove/chimney pipe. It won't burn properly; you will get smoke back and end up covering part of the feed tube to make it work.
The exhaust system must match the size of the combustion system or be larger...not smaller.
Technically, the steel drum is not tall enough relative to the heat riser to be an effective bell. If you want to read more about bells, there is a document on the blog in my signature which compares and contrasts bells vs. flues. A traditional rocket mass heater as described in the book is a flue system; the exhaust is routed through a constricted space and stratification by temperature does not take place to any significant extent. In a bell system, plenty of space is provided for the exhaust to stratify by temperature. The warmer gasses which were involved in the combustion stay in the bell until they have given up their heat to the surface of the bell. The cooler gases, such as Nitrogen (which is 79% of the air), fall out quicker and are swept out of the chimney.