well, i put in another 2.8 into my "87 berlinetta not too long ago. she vibrates pretty bad now and i don't know why. it's not too bad at idle but it increases as you push the accelerator and the whole car shakes bad at 3 grand. the engine runs nice and smooth and doesn't hesitate when i give her juice, and the clutch engages smoothly too. no grabbing or chattering or slipping. the only thing i did to it besides swap the motor was gut the catt. i'm thinking that maybe i didn't tighten the motor mounts enough. could that be it?there are also a couple of them little plastic vaccuum hose broke but i don't think those would make it vibrate so bad...can anyone help me?

 

Iwould go ahead and fix the vacuum lines . also check to make sure you have the plug wires correct . you may have two of them crossed.
another thing , I believe that some of the 2.8 were internally balanced so make sure you have the correct harmonic balancer on the engine.
Just some things to look for.

 

TTT

Fix Vaccume leaks. Check the motor mounts. If you did not tighten them enough you may have broken one. Or maybe it is just loose. It could also be your timing is off. just a few things to check.