I replaced the tbi 350 in my 92 chevy truck with a new stock gm/goodwrench 350 crate engine. I reused my flywheel and intake manifold, and a new clutch kit was installed, a new harmonic balancer was already on the engine, installed by the factory. I had no vibration problems with the previous engine, but now at 2500 rpm there is a noticeable vibration that fades out around 3500 rpm. It is not terrible but noticeable as it runs much smoother at lower rpms. Any input is appreciated.  

can you feel it sitting still??? or driving..if driving did you lower the truck.. it might be the pinion angle to the driveshaft is wrong..

if you can feel it sitting still can verify that this was a 350 and not a 383 or 400 that might require a weighted flywheel...

another thing... was there a pilot bearing installed??? or did you install one.???? that would allow the input shaft to get off center and viberate like you describe... maybe even worse...

wayne  

Did you have the flywheel ground when you changed the clutch?

Check the obvious, look for a bad motor mount. The harmonic balancer is the same for both engines, so is the flywheel, so it's not a mismatch of parts.

Also check the pilot bearing, that could be another possibility.  

Yes, I can feel the vibration sitting still, clutch in or out. The engine/truck is stock with 158K miles, I had to give the dealer the VIN to order the engine so i'm pretty sure I have the correct engine, so flywheel being corect is not the issue. I did not have the flywheel turned or ground while swapping engines. The new motor had a needle bearing type pilot bearing in the crank so that shouldn't be an issue. I am going to pull the tranny this weekend and have the flywheel inspected. Thanks for the input.
spencer