Off the AA headers I used 2.5", wrapped the passenger side around the front of the oil pan to the drivers side, and ran the two into one 3.5", and the straight and then a muffler and out the back. Routing a true dual exhaust gets to be complicated with how our trucks are, so the single is probably the way to go. I could easily put a cat in between my Y and muffler, as my muffler is almost in the stock location. I can take some pictures in the light tomorrow, but I think there are some in my build thread in my signature.
1976 TLC, did you get away with only one cat in cali? I seemed to think you needed both. using one would be much easier then squeezing 2 in there.