Dave,
The fittings that I've purchased from Lowe's are marked in the PVC with either (or both) D-2729 and D-3034. The socket in these fititngs should be at least 6-1/4" (but not more than 6-5/16"). Both the white bell-end D2729 and light green D-3034 6" PVC are very close to exactly 6" inside diameter and have walls that are 0.10" and 0.125", respectively, which maxes at 6-1/4" outside diameter. Both types should fit (the bell ends should measure about the same as the socket of the fittings). If you somehow have fittings that are thicker, they may be Schedule 40 and won't fit.
If you have the correct fittings and the pipe still doesn't fit, then you can try slitting the pipe about 4" back with a saber saw, making about 3 cuts. If you can make two-three disks of 3/4" plywood exactly 6" in diameter (or even just a little smaller), use them as an inside form in the end of the pipe and pull the pipe tight to them with a 6" hose clamp. Then use a small amount of PVC cement to glue the saw kerfs. You may need to sand off any excess cement. You may need to experiment a little with the width of the saw kerfs, but this will reduce the pipe diameter the amount you'll need.
DO NOT GLUE YOUR DUCT JOINTS! Why? Because you will inevitably want to change something and it is not necessary. If you want to "hold them in place", use a small amount of clear silicone caulk on the outside of the joint after it is assembled.
Hope this helps
