I bought this item on Amazon two years ago:
Titan Attachments Ballast Box for 3 Point Category 1 Tractor and Loader Hitches
4.4 out of 5 stars 111 ratings
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800 lb. Capacity | Fits Standard 3 Point Category 1 Hitch
Lower Lifts Pins and Top Link Pin Included | Sliding Front Door
Use With Sand, Steel, Concrete, Gravel, Rocks, etc. - Integrated Tool Holders
Volume: 5.3 cu. ft. | Finish: Powder Coated
Box Dimensions: 25.5" x 24.25" x 15.75" | Weight: 132 lbs
2" hitch ball receiver makes it easier to tow while using(Hitch ball not included)
It sounds similar to what you're getting, though "Titan" and "Titan Attachments" are two different companies IIRC. I caulked and sealed some interior seams and then put two large pieces of scrap steel in the bottom and filled around them with concrete mix, and used pourable self leveling concrete for the last pour after carefully leveling the box on my 3PT with boards cribbed under it to support the weight. When this had cured a week or two, I poured liquid Flex Seal (the kind that comes in a paint can) over the concrete and painted it up onto the inner sides. I think I have a perfectly sealed assembly that wouldn't allow water in to rust the box out from the inside. I carefully calculated everything to bring the total weight up to 800 lbs, its rated capacity. It's not filled all the way to the top, there's about 4" of depth still available, which makes for a storage tray (I keep my rubber Harbor Freight wheel chocks in there). I don't think I could have hit 800 lbs even completely filling the box with concrete, if I didn't have those big steel blocks in there. I think they were about 18" by 24" by 3", as I remember. I also put a solid steel bar across the top on the center line, with an eye bolt so I can lift this with my overhead hoist. An 800 lb box is pretty difficult to move otherwise.
If I had your lead block, I'd take it to a recycler. Lead that big is valuable.
Your original question boils down to: is there a downside to putting a block that's 25% heavier than the rated limit into your box? Well, yeah. I'd always be worried the next bump would be the one to wrench the pins upward.