Here's the rub, as I see it. No matter how competent you are as a welder, you are not capable of getting your custom-made hitch rated for road use. Even if your hitch would hold a million pounds, if you are in a trailer-related accident, there is the possibility of the insurance company or the DOT saying, "Your trailer was over your hitch's weight rating." And when you come back and say, "No no no! I beefed it up!" that is not going to carry any weight at all with them.
Now, I don't know what model year your truck is, but mine is a 2005, so that's what I looked up, and
here is a link to a hitch for my truck with a 2550 lb tongue weight and 17k lb trailer weight rating, and it costs $295 plus shipping. So if you could beef up your existing hitch for $150, this will cost you $150 more than that, minus the value of your time and consumables, and you will be good to go. Me? No question which way I'd fall.
Whatever you do, please be aware of the requirement for a weight-distributing system on many trucks' hitches. I find that a lot of people aren't aware of this, so if you are aware, then just forget I mentioned it. My truck is rated to tow 13k lbs with the factory hitch, but whoops! In the fine print, it says that you can only go up to 3,500 lbs or 350 lbs tongue weight unless you have a WD system installed. I can't speak for your truck, but if the hitch maxes out at 10k, I would bet that assumes a WD system, and the non-WD rating is more like 3.5 to 5k lbs. I know some factory trucks, like the Tundra, and some GM trucks, achieve max tow rating without needing a WD system, but they're in the minority.
Also, it's surprising to me that your truck has a 10k tow rating, but only 500 lbs max tongue weight. A bumper pull trailer needs 10-15% of its weight on the tongue, so a 10k trailer with only 500 lbs on the tongue (5% tongue weight) would be dangerously tail-heavy. Maybe the 10k rating assumes a gooseneck or 5th wheel, in which case the tongue weight would not be applied to the bumper hitch. I dunno.
