vibration

Hey everyone. I'm new to the forum and am hoping to find some answers (other than the several I have gotten locally) on a vibration issue.
My car is a 74 Dart Swinger with a 318/904 and 8 1/4 w/ 3.90 sure grip.A few years back I rebuilt a 318 with forged flattops (can't remember brand or find paperwork) stock port J heads, Comp cams cam (280/480), LD4B intake, Holley 600, Hooker Super Comps, orange box ign, stock balancer, TCI 10" converter w/ Mopar 7/16 hole flexplate. Timing @ 32* full mech.
From the word go, this setup has had a vibration that is not evident at idle but increases with rpms. It is worst between 2000-2400. It vibrates when in park as well as when driving. I actually drove this for a few years like this and nothing ever came apart.
In the spring of '07 I pulled the motor to have it freshened and finally get rid of the shake. The machine shop told me everything showed normal wear and no signs of anything abnormal. I also had the tranny rebuilt telling him about the vibration. He also found nothing abnormal.
When everything came back, it all went back in the car and it didn't take long to discover I hadn't cured the problem. I then proceeded to try every possible balancer/flexplate/converter combo I had access to and nothing worked.
Spring '08, a guy I work with offers me his bone stock 360( w/ 75 casting date) until I can get mine staightened out. So I drop it in w/ a b&m 360/904 flexplate and the vibration remains.
I've talked to as many local mopar guys as I could find locally, but obviously to no avail. Any and all suggestions would be welcomed and appreciated. Sorry to be so long winded, but I wanted to give as much info as possible. Thanks