1975 L48. Need to put on a complete exhaust from the manifolds back. From what I can tell the L48 and the L82 both had the same exhaust manifold/heat riser valve. The L48 had 2" exhaust and the L82 had 2.5" exhaust. I wouldn't mind using the 2.5" for a little more flow. Is the connection to the heat riser valve the same or do I need to get some type of adapter to run the bigger exhaust with the stock manifolds/heat riser? The catalogs are a little vague on the type of connection to the heat riser and show a donut ring in the connection to the heat riser but don't explain if there is a different one needed for the bigger exhaust.
Thanks

The stock exhaust was 2" at the manifold, and the pipe size was expanded after the flange in the earlier('68-'74) cars. In the case of the '75/later "Y"pipe cars, the pipes after the "Y" were 2.5". If you go to a 2.5", true dual exhaust, the head pipes will be 2" where they bolt up to the manifolds, then swedged/adapted out to 2.5" just a few inches past the flange connection.
So, your manifolds & heat riser will still be fine to use as is.