IMHO the head has to come off.
The clearance between the intake valve and piston @ TDC is less than your lift, which means if the valves really stayed full open all 6 intakes hit the pistons. The exhausts have plenty of room, and won't make contact.
Most likely you'll just have marks on the domes, since the cranking speed is low and the valves are nearly vertical.
If you're not lucky the stems are bent.
