That cart doesn't have a single tree. It has stationary hooks hooked on the shafts about the 76" mark, That is where you are to hook the traces.
I believe this cart is too small. I don't know what they are asking for it, but I think they do not resell well.
I would try and get one that fits and hopefully you will not need to resell.
If you do get this cart, a cart without a single tree is hard on the horses shoulders, with a singletree the breast collar moves with the horses shoulders, without a singletree the harness rubs across the chest and can cause sore shoulders. That stated you would be best if you made a single tree for that cart. But of course it does not change the size problem. Those kind of carts are really for ponies only.
Glad you are repurposing your boy, good luck to you.
The best time to plant a tree was 20 years ago, the second best time is today.