I've fabricated a few hitches, and installed some factory built hitches, and have 100's of thousands of trailer towing miles. Not necessarily an "expert", but been there, bent that.
Without seeing your bumper, it's hard to say whether it will take a hitch as is or not. I have seen step bumpers of the type you describe built plenty strong enough to take a hitch, and others not so much. So just a few offhand thoughts on that, if you are welding directly to the existing bumper don't trust just welding to that 1/8" plate, at a minimum you need some substantial gussets to spread the load (in addition to welding to your c channel), 1/8" is not really all that heavy for a hitch mount. Even welding to your center channel I would consider a couple of pieces of box tubing running diagonally up to the actual frame rails of the truck to give more rigidity. I think when it comes to a hitch you can't really make it too strong, if a car to trailer gets loose it is big trouble. Also, when you put in the brackets or loops to connect your safety chains, weld those someplace besides the receiver tube. I always felt safer that way so if there is a problem with the tube itself the chains are hooked to something else. And use something substantial for the loops, remember on the one time you really need them they will take a helluva jerk when the trailer is loose. I've seen guys (no, not me this time!) with the trailer off the side of the road complete with the eyebolts that used to be attached to the truck bumper still nicely clipped to the chains that jerked them loose.
Other issues, i would try to keep the height to within the ability to use a 6" drop ball mount if possible. Anything over that gets expensive real fast, and also the farther you drop the more leverage the trailer has to try to twist the truck hitch around. Also if you ever need to use a weight distributing hitch there are limitations to how far you can drop with the mounts available for that type of hitch, and in that case I know from personal experience that the required solid forged ball mount can run $125 or more just for the longer drop in addition to the weight distributing hitch itself. Also, the rated "drop" on a drop hitch is measured from the top of the tube down to the top of the ball mounting surface, and if inverted for a raise, it is measured the same way from the top of the tube to the top of the mounting surface in that position. That little tidbit could have saved me some confusion back in the day.
You mentioned the pin hole. One thing to consider is access. You could do a real pretty job of getting the tube all nice and recessed into the bumper out of the way and find out you get real tired of getting down on your knees in the slush trying to stretch your hands up inside the bumper to put the pin in by feel. Ask me how I know. lol.
Also you may want to consider turning angles where the trailer could potentially come in contact with the trailer when turning sharp. I am assuming your bumper is substantially wider than a normal pickup bumper, so the closer the ball is to the truck, your turning angle is reduced limiting how sharp you can turn without hitting the trailer. Letting the receiver tube stick out farther, or using a longer ball mount helps that situation. Particularly if you ever have the need to jackknife the trailer into tight spots (which I have to do all the time).
I don't have any personal experience towing a toad, but from talking to people who have they can really put a lot of stress on a hitch from time to time. Case in point, once in a while, usually in parking lot situations, apparently the steer wheels on the car can decide they want to go their own way placing a lot of side load on the hitch you would not necessarily see in a trailer towing situation. No really sure if that needs to factor in, just thinking out load.
OK, one last idea. I've seen some nice steps (single or double or just a toehold) you can buy or build which slip into the receiver tube giving you a nice leg up climbing into the truck, which may also have the added benefit of being something big enough to keep your attention and reduce the shin injuries. I even saw one that folded up for driving so you don't have to remove it.
Good luck!
Dave
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