Todsy I set to and tested my welds for strength. That was actually easier than I had envisaged. My original plan had been to attach a chain to the loop I'd welded then suspend the plate with the chain dangling down then attach a step to the other end of the chain and simply stand on it.
The barrel when full of water will weigh 125lbs.
The turnbuckle is good for 130lbs.
The chain is good for 520lbs
The connectors are good for 600lbs
The weakest point is clearly the turnbuckle which is itself strong enough to support the barrel all on one turnbuckle.
There are three chains in use. I've designed the system with redundancy in mind. One chain can fail without disaster ensuing. Thus, the designed stresses are as follows.
Each barrel hanger will carry 1/4 of the weight, i.e. 32lbs. In a sudden jerk caused by hitting a pothole then I'm guessing 3 or 4 times the weight will be exerted on the mounts. That's 120lbs. Now I'm not going to say how much I weigh but I jumped up and down on that bar and I weigh over 120lbs. I'd say that my system is going to be safe.