Just dropped a rebuilt 350 in my GTA and every cold start I get a heavy metal vibration as the rpm is rising from about 800-900 and then I never hear it again. Any ideas? It affects the rpm rise so its definitely in the drive train, I think.
Question two, I have checked and rechecked vacuum lines and still get not better than 15 psi, with good idle and accel. Do I really need to have 20? (stock setup).

 

Are you sure the flexplate to torque converter bolts were tight?

 

I had a second party jamming the flywheel so I could guarantee tightness with both hands. The machine shop suggested the torque converter itself might be vibrating, has anyone had that?

 

Yes. I used to eat converters on an old Powerglide. It seems the stator would start breaking free, but there are lots of other possibilities.