I hate to say it since you seem to be determined to use item C, but I don't think you should be using it at all because:
- It is too wide. Even if it doesn't bother you most of the time when using the loader, it might when you are getting in and out of the shed or in the woods , etc. Sticking out three and a half feet behind your 3 point and 6 inches out to the side, it will clear a big extra path behind you as you turn and manouver. It will be a problem eventually, even if not very frequently.
- It is not balanced right and will locate too much of the weight out too far on the forks, causing it to tip away from the 3 point.
- It doesn't have any support in the right places relative to your forks, so you will be welding the forks to the little bars instead of the main support members.
- I am concerned that those forks will not be strong enough without a triangled support bracket going from the middle of the fork, up to the middle of the back plate. I can't see if the bottom of the back plate is I beam or C channel, but it looks like as soon as you put some weight on those forks and start bouncing down the road, it is going to beeeennnnnnnddddd. Item C will get in the way of that support bracket.
- It is going to spread your cement over such a wide area that it will be thin and prone to cracking from the bouncing, raising and lowering, etc.
- You don't have a welder, so why try to weld it there in the first place if it isn't helping the design?
There really doesn't seem to be any good reason to include it. My suggestion is to just build the box out of 2x6's or 2x8's to fit item B, then fill it with the required cement to give you your ballast. Using 2x's will allow you to use U bolts or straight through bolts to attach to the forks as well as the back plate, and support your box better without welding. Make the box sides high enough to put your tools or wood in after the cement is poured. If you want to find a way to weld or bolt on a corner brace to support the forks a bit better, it would help too.
You already have another use for item A and something will eventually come up that is appropriate for C, never fear. Perhaps you can use it to make a little trailer for your lawn tractor or something.
My two cents.