You should have the rotating assembly balanced and you need to use the 400 balancer and flywheel or flexplate. You may want to think about buying a balanced rotating assembly, all the parts will work together and you can get a good price in a kit. If you use 350 rods w/ custom pistons you my want to consider using a small base circle cam or you will need to check the clearance between the rods and the cam lobes. You can use a piece of bent coat hanger to gauge the clearance you need between the rods and the cam and block. Hope this helps.
Sean
