Nobody is sayin you definatly need a crank, were sayin pull motor, split cases and have that crank verified that it is still good and true, then get it welded to prevent possible seperation, more so critical when you add porting and compression.
I rebuilt my motor a couple years ago, damn sure wasnt gonna slap a new top end on it without the crank being checked, everything LOOKED fine, come too find out both rods were bent and it had a bad bearing. Couldn't tell it had any issues at all. How do you think that would have held up if it wouldnt have gotten repaired properly?