Excellent question!
The simple answer is Yes, a valve would be open, but in order for the pressure to escape, two valves must be open on the same cylinder. This only happens on the overlap period when both intake and exhaust valves are open.
The stock IDI cam has 38* of overlap. Multiplied by 8 cylinders = 304* of overlap.
Since the IDI is a 4 cycle engine, it takes 2 crank revolutions or 720* for this to happen.
720 - 304 = 416. So there are more non-overlap degrees (416) than overlap degrees (304).
You just have to rotate the crankshaft to a non-overlap position. That's the theory.
In practice, when you apply pressure to the intake manifold it will force the piston with the open intake valve down. This will be counteracted by the cylinder undergoing compression.
The engine should quickly find its equilibrium.
Worst case you might have to manually rotate the engine to a non-overlap position.