Nice pipes! I've been considering 3" mufflers and tail pipes for my '76. Not sure about ground clearance with the large mufflers though.
Pittsburgh, Jim and Tom Hand have done some extensive exhaust system testing. Not only have they tested mufflers but they also tested crossovers. Check
http://www.boyleworks.com/ta400/psp/exhaust.html for the complete run down. The X is unlike a Y in that the X branches back out into two separate pipes where the Y necks into a single pipe.