When you were describing the build my thoughts were, "this is going to take 2 men and a boy to get in place". I have one like in the first picture and it will hold way more than 300lbs. I hoisted a wild hog that was closer to 400 and it was work, but the frame didn't squeak. Sooner or later you MIGHT approach the limit of your truck receiver, so just be aware of that number as well.
Someone beat me to the recommendation of an extra pulley in the line. Just anchor it back at the top near the bottom of the last pulley and it will make it easier. I would weld a D ring or similar up there.
Those hoists are great for many uses.
I would try the extra pulley and some test weights before spending the cash for the wire. Doubling the capacity, will reduce the amp draw as well, so what you have might work out real well. One thing to remember, is it also reduces lift speed by 1/2. I would test it before removing any cable as well. Adding a pulley, doubles the amount of cable needed.
One other way to add support, something I have thought about, is movable and adjustable leg supports for the hoist frame. That takes a LOT of load off the receiver. I used to use mine for loading and unloading the 5th wheel sliding hitch from my truck. Lift it right up over the side of the bed and set it in place.