You’re close to OK. I have used the replacement box on 28s without problems. They all are prone to rustout because of spilled battery acid and road moisture.
If I remember right (it’s been a while, I’m old, and the memory is weak), the box is bolted diagonally, one corner at the frame and one corner on the crossmember. It is designed and proportioned for (and
only for) a group 1 six volt battery. The original ‘28 setup was the battery was held down with two corner brackets similar to those shown in your picture. Later a full four sided frame was used and bolted at the same diagonal corners, and that is most commonly available and currently used. Don’t neglect these top bolt-down items, whichever you have. They keep the battery firmly in place, which helps minimize acid splashing out.
I hope I have most of this right and haven’t steered you wrong.
You’re fortunate to have an original 28 battery cover. It fits snugly with those spring shaped things into a corresponding oval cut into your floorboard for battery inspection beneath the driver. Are your floorboards also original, that is, solid wood with original support bars underneath? These original items are hard to find and worth keeping and restoring. It would be worth your while to buy the MAFCA/MARC Restoration Guide and Judging Standards, even if you never plan to be judged. They will answer many of your questions and save you a lot of confusion, misunderstanding, heartache, and money. Trust me on this.
