When the crank has been finally pressed together, we’ll need to check the run out. Ideally this will be zero, but is something that must be checked if longevity is going to be achieved! (And I do plan on the engine lasting!).
Minor runout could be adjusted with subtle use of a mallet, or for anything more serious machining one shaft. Or even making an eccentric sleeve. But hopefully none of these will be required.
The idea of the v blocks is to mount the whole crank assembly in the bearings on the timing side. The lot can be spun by the conrod and a clock on the other side measures any runout. Easy!