It sounds like you may be installing the connecting rods backwards. Look at the rods down where the two rods would be riding together on the crank journal. You will notice that one side of the rod has a slight bevel and the other side is machined flat. The beveled side must go on the outside against the crank, the flat side goes next to the adjacent rod on the journal. The more rods you install the worse the interference. If you find that you do have them backwards, depending upon what type of pistons you are using you may have to pull out the piston pins and turn the pistons around as well.