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.

One of the ways OMC got the extra five horses in ’56 was by adding needle bearings to the wrist pins. Early testing did not show any problems and so those needle bearing connecting rods were used in the first production run of the ’57 35. One thousand hour wide open testing of the prototypes ( A standard instituted by Ole Evinrude in the 1920s ) had not shown any problems. The first production run of 5000 outboards was made in November for Christmas sales and December boat shows. Sales to Florida and Texas were particularly important because early use in those warmer regions constituted "real world" testing outside of the engineers laboratory. Spring sales were well under way when reports of wrist pin failure hit the company. It was found at high speed the needles circulated and stayed centered, but that in as little as 200 hours of trolling the back and forth rocking wore through the retainer and dropped the needles into the powerhead with disastrous results. The bearing specs were changed and needle bearings with a heavier retainer were used for the rest of the year. Ralph Evinrude had 5000 of the newer 35 horse powerheads sent out to dealers for warrantee replacements of the defective outboards. The dealers were supposed to arrange for this replacement with there customers and letters were sent out to registered owners. It was found many did not read or chose to ignore these letters. Some were pleasantly surprised when the outboard failed five years later to find that they got a new powerhead under there one year factory warrantee . . . 😎