Okay I've seen a couple of different designs for down pipes. Some have a bell shape; some have a flat flange; some have a smaller tube running down along side then merging into the main pipe. Where can I find out about these different types? Which one is best design?

 

IMO, a pipe is a pipe. As long as its 3", has decent quality/welds and fits your application, it should be good to go.

Most popular are Bellmouth and Divorced Wastegate designs.