Hey Dave,
The distance between the end baffle or end plate and the second baffle is 6.5 inches. I left the edges of the first plate 1 1/2" hole at that diameter to act as a "restrictor" rather than cut it back to the full diameter of the pipes themselves.
Baffles not only quiet it down but help generate back pressure to heat the exhaust to add in combustion which factors in the process some way. But even if this was bad advise (From Helicopter mechanics) I chose not to get too close and risk damaging the outer wall of the pipes. I shot some black heat paint up there and darkened it up and it looks great, sounds great and has worked great for me. The tone issue mentioned is the real treat and the tone drops with each baffle you tap into. But I stopped at two as in the picture below.
Baffles 
