I found if using clamps they are too big and foul on the upper crankcase. I've always fitted them without clamps, the taper is normally enough although I compress the rings using two coffee stirrers as the piston ring enters the taper of the individual bores. Use plenty of oil on the bore at the taper end and plenty on the piston and rings. Go very gently and have patience, just tap the cylinder block down over the pistons with a gentle tap using the palm of your hand.
I've never broken a ring using the above method.