My '78 was orignally a 305 automatic, but I converted it to a 4speed with the bigger flywheel setup. I had a 350 and a 400 motor. I installed the 350 because that was the flywheel I had. Since then I have heard of balance weights you can buy to change a 350 flywheel to 400 balance. Does anyone have any experience with them? If they work ok, I may eventually install the 400.

If it was me I'd buy a 400 flywheel that is specifically balanced for it. A flywheel ain't somethin' to screw around with.

I agree that messing with standard flywheels is serious business. Here's where the idea came from.
I was making some time runs at the local strip with my '96 Impala SS and had all 4 doors blown off by a scruffy looking 4 door '81 Malibu. The car ran mid 12's and I talked to the owner in the pits. It had a 400 4 speed, and he said he didn't have the right flywheel, but his engine builder had installed a purchased balance weight to compensate. He didn't know the make of the balance weight.
Some time later my Impala was having vibration problems and I was looking in Carparts.com for the price of a torque convertor. http://www.carparts.com/parts/sample.asp?reflink=0-0-0-0-0&SegmentId=1&parttypeid=23015&vehicleid=57918&catalogid=&partnum=NONE&dc=

I don't know why it was listed under Impala SS/replacement/torque convertors, but it listed a "Balance Plate to convert 350 Chevy std. flywheel to approx. 400 (S.B.)balance .. Sealed Power... 416-002 ...$20.49" They also had ones in 454 balance.

It sounded like an inexpensive solution, but I was hoping to get some specific brand feedback before I tried it.