You can't 100% measure the crank in situ to check for ovality/roundness in the same way its done at a machine shop, I agree there. But you can measure the diameters of the journals a few times across different planes and if they all measure the same, it stands a very high chance of not being oval.
Thing is, taking the crank out involves either engine removal or at least gearbox removal, so for a lot of cars the only two fiscally-sound options are 1) to check/measure crank as best as possible in situ and if it appears OK, fit bearings, 2) scrap the car.
Yes it would be better to take the engine out and do a full rebuild with all new bearings, regound crank, re-honed bores, re-faced valves and cylinder heads, the full works. But sometimes a "patch up" is the only thing thats worth doing on these old cars.
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