First post: neutral rattle

Hello Forum members, this is my first post on this forum. I have been a member of slantsix.org for awhile known as Slantfin. I started to feel guilty about sneaking in questions about my 340, so I decided to join For A-bodies Only. My problem is that when in neutral, the clutch rattles and makes all kind of noise. Engage the clutch and it quiets right down.
I just finished a ton of work, which makes everything harder to figure out. For starters, I welded in a transmission crossmember that was chopped out to fit in a Doug Nash 4+1 manual trans. I also replaced the motor mounts and transmission mount.
Since I started the car after re-installing the original clutch and heard the noise, I've installed a new Centerforce package, re-installed the original transmission mount, which was modified to fit the DN, and adjusted the clutch linkage to 1" of free play. When I bought the car 15 years ago it had a Centerforce clutch, so clearance shouldn't be the problem. Sorry: long post. Ideas, anyone?