There's no good reason to buy a 500lb actuator when a 1000lb unit is available. You need to consider the actuator's ability to resist simply failing and pulling apart from the extension loading exerted by the pallet. Your assumption that the lift links are carrying the load is incorrect. Prove this by attempting to lift the rear of a full pallet enough to slack your existing top link.
While I think something less than a 1000 lb might work I would not get less than a 1000 lb. The difference in price between 500 and 1000 is not that much. An AEI 1000 lb actuator with an IP66 rating is something like $170. I don't think I could do it hydraulically for that, not to mention having to mount a valve. The test you mention would be easy enough to do just to see how much it is loaded, out of curiosity. As I am thinking of this test we would have 2 different loads here, I think. If I do as you say and lift the rear of the pallet, this load will not be more than half the load of the pallet and forks. Lets say my forks weigh 100 lb and an evenly loaded pallet weighs 400 lb for a total of 500 lb. If I lift on the rear to loosen the top link I think it would be nearly half the total of 500 lb with the other half supported by the lift arms. But that force is a vertical one. The force at the top link is a horizontal force which I am thinking would be harder to measure. I'll have to think about that the next time I put the rear forks on. Maybe we have some engineers out there who could enlighten us. Anyway, some actuators have a static rating 3 times that of dynamic; that's why I was thinking a 500 lb one might work, but I wouldn't spend the money to find out.