Reality is, ratings have some science to them, along with how much warrenty the manufacture wants to get consumed by. There are items out there with one time uses.....Others have multiple uses. A one time use item is usually smaller, will break or bend easier than a multiple use item.....
Hence why Ben probably uses the "do you believe or not" comment a lot! Or items have multiple ratings. Chain for example, a lift rating, or pull rating. Pull is many times 2-3 times the amount of the lift. My bobcat for example, the lift rating is half of the tip load, ie where the back wheels lift off the ground. I can push more than tip load, which I do with pallet forks at times. Full well knowing I can not lift the load per say. Safe?!?!? I'll let you decide.
Legal, as noted by someone in Virginia... he has 8K paid for registration, that is what he is legal to. 7200 lbs gvwr door sticker be danged. Washington where I am, is very very similar. My C2500 has an 8K paid for registration, as does my sons toyota tacoma, and other sons K1500 reg cab pickup. Door stickers of 6200-700o or soo be danged for sons rigs. My 8600 also be danged. If pulled over between 8001 and 8600, I am illegal thank you very much.
My IHC mdt dumptruck, has a door sticker of 18200, sits inside my owners manual on office book shelf. I have a paid for license of 26K. Been thru state weigh scales upwards of mid 25K, never been ticketed for being over weight. Even got pulled over at 27xxx lbs.....no ticket, a 10 day warning to up paid for license to 28K. As I was under the max axel load for tire width so I was not over the road bed engineer ratings. THAT is what the LEO/CVEO is supposed to ticket you on, assuming you have enough paid for registration.
As one CVEO told me in a class I took about what I have to register my trucks for. I could license a Toyota tacoma or equal for 100K lbs, drive 100 miles down the road, truck is destroyed, haul it to junk yard, nothing they can do IF I follow ALL other laws from braking to min speeds etc. This would be in this instance, a one time 100 mile use tool! Many of you may not agree, but it is what it is!
Reality is, how long do you want something to last, before breaking etc.
Rig in question of OP's, probably could be legal rated paid for to around 20K gvw. Issue will be, if it can stop in 25' or so from 15 or 20 mph, or what ever spec and LEO uses in the field to see if your brakes function or not. Pass field test, your legal. Fail.... well, you get a ticket, vehicle does not move on road until you pass test, or haul it to shop on a vehicle that can carry the load legally etc.
Also, from a changing door sticker gvwr etc. Usually easier to do if you started with a cab and chassis model, put the bed on aftermarket. A pickup while it can be done, it was assembled at the factory complete, and most aftermarket body manufactures etc will not put there insurance on the line for these additions. There are a few that will, but many will not. Kind of like the two places that put aftermarket front drive units on Vans, they will usually only do NEW unregistered vans, not used ones.
Good luck on what you are doing.
marty