Two things you need to be aware of when you start talking about extending hitch points out from the vehicle...
As you've already mentioned, it will reduce greatly the tongue weight that your vehicle can handle. The added length increases torque effects exponentially... Think a "breaker bar" effect. If you can't budge a nut with the regular socket wrench, then a 2' breaker bar will get it off with little effort. The same physics apply. A relatively light 350 lb tongue weight can rapidly become 1,000 lbs in short order just by lengthening the fulcrum rod.
Not as obvious and usually not even thought about. Lengthening the distance between your trailer's hitch point and your rear axle does bizarre things to handling. That lengthening not only affects the torque of the tongue's weight, but, it amplifies any handling issues that you may have. While your tow, now, may be "rock solid", it may be that your truck's mass is overpowering the trailer's ability to destabilize it. Lengthening that distance from axle to hitch point also increases the ability of the trailer to destabilize your truck. In essence, it will amplify not only the leverage force down (tongue weight), it will amplify any yaw (side to side) motion that may be controllable now, but, won't when you lengthen the hitch point. That's one of the reasons that 5th wheels are so much more stable...the hitch point is directly over the axle. The Pro Pride and Hensley hitches are so highly touted because they do the same thing with levers...they move the hitch's effective mount point from the bumper area to closer to the axle. Along with the sway potential increase, you will also find that the trailer will react quicker. A result of that same sway increasing length. A 5th wheel reacts much slower to steering inputs when backing (I'm still getting a handle on that!
) because there is zero "lever length" with a 5th. You may find that backing your relatively short trailer may become a bit of a chore.
Just some additional things to consider before lengthening that "lever" that connects your box to your truck. The tongue weight on your relatively light 189 should be manageable, but, if you are towing, now, without sway control, sway could be an issue, if you are towing with a WDH with sway control, you may need to readjust your settings. Either way, handling (backing) will not be what you are used to...