My Johnson Common Parts list shows the same crankshaft part #376380 was used on the QD-15 through the QD-24 motors so your 1956 and 1961 crankshafts should be samey-same. You are correct that the rods have to removed so the pistons can come out the head side of the motor, so since you need to pull both powerheads apart you can just compare which crankshaft appears to be in better condition once you get them torn apart. Examine the crank and rod journals for pitting or scoring and take some measurements to check for excessive wear.
Bob
1937 Champion D2C Deluxe Lite Twin
1954 Johnson CD-11
1955 Johnson QD-16
1957 Evinrude Fastwin 18
1957 Evinrude 3022
1958 Johnson QD-19
1958 Johnson FD-12
1959 Johnson QD-20
1982 Evinrude 25hp
“Every 20 minute job is only a broken bolt away from a 3-day project.”
"Every time you remove a broken or seized bolt an angel gets his wings."
