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These motors are often a little loose as you described. 1956 was the first year for a needle bearing at the wrist pin end of the rod. They were and improvement over the bushing from 1955 and earlier, but still too small. Turn the motor so that the rod is pulling the piston down the cylinder. Then stick a screwdriver through the plug hole and gently push down on the piston. If you hear and feel a "click" then you likely have slop in the wrist pin bearing.
The big end of the rods can be checked by measuring the crankpin diameter and the inside diameter of the rod bore. I find that as long as the needles on the big end look good with no galling or bad discoloration from heat, they will usualy work just fine.