correct its basically end float on the crank caused by the weaker main and end bearings in the block.

i has a 4 recently ran it at 360 with no problems at all and had a fair bit of abuse including at the 'ring a few times. an early sign is the crank pulley separating from the rubber damper. as mentioned from before if your building the just go forged or for a little cheaper fit 7 or 8 parts will handle 400/400 no problem.