Moving that weight further behind the trucks rear axle will increase the effect it has on payload and may overwhelm your WD system. Going from 750 pounds of TW to over 900 will mean you will need to re-adjust your hitch.
I agree with going to the scales first to see where you are. If your TW truly is 750 and the trailer total weight is 6000 then adding that much to your TW would put you at about 15% which should be great.
It all comes down to whether or not you have the cargo capacity in the truck. Moving the generator to the tongue means it is still mostly carried by the truck and by adding the cargo carrier you may actually be carrying more weight by the truck, not less. I would keep the genny in the bed and not go to all the work for little or no gain if removing weight from the cargo capacity is your goal.
