If it's close the blades will cut the radiator when you make a hard stop. How far away depends on the condition of your motor mounts - old rubber shears off pretty easily, and not all mounts are "safety" type, where there's still metal if the rubber goes away.
You can restrain the motor from moving (like rear to front) with a simple 5/32" wire cable between the block or head and the frame behind it. Weighs nothing, costs almost nothing, completely silent (no rattle like a chain), easily adjusted after installation. It's completely slack unless the engine shifts forward - then it stops completely.
I have one like this preventing my exhaust from being crushed on my 1974 Dart Sport 360" - been 15 years, still works.
