I agree with you if you were to just make it a notch or a rough blockout. It would need to be blended in smoothly over a fair distance to not get a stress riser.
It is crummy that EQ makes different attachment types for the different capacity bars. They should do one hitch head swivel design that all bars fit.
I wouldn't grind both dimensions of the bar, personally. I'd only modify the height. I would get hold of a 600lb bar and mic it's width and depth. I don't know what they are, but for the sake of argument, just assume the 600lb bars are 1" tall and .75" wide. Now assume your 1000lb bars are 1.25" tall and .8" wide.
You have to calculate the stiffness, which is very simple. The formula is I=(bh^3)/12 where I is called the Moment of Inertia and relates to bending stiffness. B is the base width, and H is the Height. So for our 600lb bars, we have I = (B * H^3)/12 --> I= 0.75 * 1^3/12 = .0625 in^4.
So now, you simply have to make your 1000lb bars equal that value.
Stock, they are: I = (0.8 * 1.25^3 )/ 12 = .130 in^4. You can see that the stiffness here goes up with the cube of the height, so making the bar a little deeper makes a huge difference in stiffness.
But anyway, what I would do is set my I = to 0.0625in^4, and then keep the width the same on the 1000lb bars, just reduce the height. So if you solve it that way, you have: 0.0625 in^4 = (0.8 * H^3)/12. You solve for H, and you get H=0.98".
So, I would mark my bars about 15" aft of the pivot point, and grind off an equal amount top and bottom. Slightly over 1/8" in this example. I would taper that out over a nice long distance, at least 8" either side.
It's the vertical bending stiffness that you're trying to match up. You could do this with a disk grinder and a belt sander at home easily enough. Just make it smooth.
You could also do like Andy suggested and just grind the whole bar down, though you'd have to leave it thicker at the head stock end where it goes into the hitch head.
You certainly do not want an abrupt transition anywhere, as that would lead to stress risers and you could fail the piece.
It would be easier to just buy another hitch. But if money is tight, this could be done at home.
My buddy has a Bridgeport mill, so I kind of forgot about having to pay a machine shop
But yeah, that would alter the equation a lot. But no more material than would probably need to be removed, I'd probably just do it with a grinder, a file, and some emery paper. Don't forget to paint it...