I am under the impression that the rating of the spring bars is related to how much upward force the bar is CAPABLE of producing. There is nothing to say that you must actually use all of the available force. In other words, you pick up only the number of chain links needed to set the weight distribution properly. And you don't pick up any more, even though the bars are capable of it.
The process for determining the needed number of links is described in the Reference Library - TM Info You Won't Find Anywhere Else.
Of course, if you have 300 pound spring bars and need to pick up 400 pounds, you are out of luck. Underspec'ed bars won't work, so you should choose bars that are a little stronger than you think you need, rather than a little weaker.
Having said all that, I should point out that there is one potential problem with grossly overspec'ed bars like mine. Occasionally you will need to cross a wide swale or drainage ditch by driving down into it and up the other side. When you do so, there will come a moment when the tow vehicle has passed the lowest point and is angled up the other side - but the trailer is still on the downslope. At this moment, there is a substantial angle between the axis of the tow vehicle and the axis of the trailer. In this situation, the spring bars should "spring" - that is, bend upward to maintain something like the original tension on the chains. With overly-hefty bars bars like mine, the spring is too stiff - the bars won't bend, and so the tension increases. With a sturdy box-beam frame such as my Explorer has, this is no problem. However, on vehicles with a less-strong frame, I suppose this might result in the frame-bending problem mentioned above.
The best answer is choose spring bars that are matched to the tongue weight, with a little reserve. I suspect that Ray's choice of 750 pound bars is just about ideal. As he has laid out in previous posts, 500-pound bars are just a bit too light for most loaded TMs.
Bill
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