I was thinking that the leak was at the head but is it at the collector or the muffler? There should be 4 "donut" gaskets one for each pipe as it enters the collector and one for the muffler. There really isn't any spacer just the three main parts, header, collector, and muffler along with gaskets, clamps, and head collars. Take a look on the
http://www.BikeBandit.com website at the parts breakdown for the 75/76 CB750F exhaust system and make sure we are talking about the same thing. Before you put it to the saw let's talk.
These two photos give you some reference as to how it is mounted on my bike. The muffler has a bracket that reaches up and attaches to the back of the rear foot peg mount.
Just had another thought. Did you get the complete F exhaust system or are you trying to fit the stock K headers into the F muffler and collector?