Hey I need some advice. I have built a 383 and installed it but noticed I have a vibration above 2000 rpm. I have located the problem which is the flexplate which is a new scat 350l sfi plate. I did not notice it was bent when installing it. But my question is since I have to pull the transmission again    which would be better to install go back with the 350L or with a 400 external balance. The rotating assy is external balance.

Thanks for any help on this.  

350 what?! If you do have a external balancer yes get a weighted flexplate, since it not going to be balanced with the existing rotating assembly it won't be as dialed in of course.    WIEDER  

You're better off with internal balancing your 383, and since it's already set up that way, just stick with it.  

I also have a 383 with a vibration when you rev it approximately 2000rpm and over. I've been wondering the same thing, that possibly it has a 350 flexplate on it.

The engine originally was in a circle track car so I'm sure it had a manual behind it. Its in a truck now with an auto and the guy I got the truck from had a mechanic install it but he's not sure what flexplate they used.

Could this make a pretty noticeable vibration? Could it  have hurt the lower end of the engine by now? (Its been in the truck for a while) Should any 400 flexplate be fine or do they balance the rotating assembly with it usually?  

The Scat 350L is a 168 tooth externally balanced flexplate. IMO,this could be the source of your of your vibration. I used a 400 external on my external balanced 383.  

I changed it to a 400 external balance flexplate and poblem is solved, no more vibration. Thanks for everyones input