The top of the box measures 10.75" x 3.75". The body of the box is roughly 10.5" x 3.5". These are known as 30 caliber ammo cans. They are made of metal and are sold at most Army surplus stores. The hard plastic ammo cans sold at many sporting goods stores are not the same. They are typically too big for this mounting position.
There is one existing hole in the deck of your tractor that is square, on each side of the deck, if I remember correctly. I used that hole and drilled one extra hole in the deck to mount these boxes. I used locking nuts and fender washers to mount them.
Make sure that you mount the boxes with enough distance away from the engine cowling to allow the tops of the boxes to stay open and not close on your hand when you are putting things in or out of the boxes.
These boxes do not interfere with where you put your foot on either side of the deck. Depending on which tractor you have, the brake pedal may be on the left side and that may interfere with placement of the box. The brake and treadle pedal on mine is on the right side of the deck and the box does not interfere with either one of those pedals.
Anyway, here are some pictures.
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