Re: Any truth in it?
I've never heard that (specifically) myself, but like Jeff, saw this model, in all it's different versions as a "motor to avoid".
The 1970/71 model 400's (the 400 was only sold those two model years) introduced Merc's (new) 33.3 cubic inch, internal reed, 2 cylinder block.
The block itself stayed in production until 1989. Over that time it was used on the model 400, 402 and the 35 and 40 twins.
I never particularly liked these model 40's and Merc did play around alot with them trying to "get them right".
They started life with Merc's relatively new Thunderbolt 3 ignition (modular predecessor of the T4) and a sidebowl back-draft carb.
When they changed it to the model 402 they regressed and stuck on a phasemaker ignition.
In 1975 they upgraded that to the T4, but still retained the original carb.
In 1979 they renamed it simply the 40 Merc without making any changes.
For the 1984 model year they renamed it the 35 (which reflected the change from rating horsepower in BHP to SHP).
When the last 35 twin rolled off the line in 1989, it was still using Merc's oldest carb design.
But back to your original question - I checked the parts listings. The crankshaft for the model 400 sold for $390. The (new) crankshaft in the 402 first year of production sold for almost $800 - so either there was significant inflation between 1972 and 1973, or maybe there was some truth to what you heard