Did you check the piston fit in each cylinder when you put the engine together? Did you check the ring end-to-end gap, getting the ring set correctly in each cylinder? Did you use cast iron or chrome rings? I've seen some very sloppy machine work done where end gaps were nearly .030" on brand new rings and that also means the pistons will be too loose in their bores. Ideally the ring gap should be .008~.009". Chrome rings are always more difficult to break-in than cast iron rings, by the way and it may be a long, long time before they seat correctly.
To seat your rings, you might go from 30 to 60mph about 6 or 7 times. That should take care of that.
