Hi BigBill. It won't be an easy hook-up. Your first obsticle will be the pto shaft speed and direction. The Kelley should be a self contained unit, which means it already has it's own pto driven pump, probably setup for the standard 540rpm clockwise rotation(looking from the rear of tractor). Your Cub has the counter-rotation 1800 rpm's pto. You'll have to do some engineering and fabrication to make that switch.
The second obsticle is the actuall size Kelley you're looking at. They make several sizes. You stated it was small. Anything over 900 lbs will be too much for the cub, and at 900, you'll lift the front end of the Cub off the ground with the bucket extended outwards. Kelley makes some units over 1600 lbs. You'll definetly have to fabricate a front loader to counter balance the backhoe.
It can be done, but it's alot of work, and not to be attempted by the average Cub owner
I'm not familiar with the lo-boy 3 point hitch lifting capacity, but the 3 point backhoe attachment hangs the unit farther back on the tractor than a rigid unit and will probably be too much for the cub 3 point set-up. My unit is rigid mounted and as close as possible to the rear end.
Kelley's are similar to the Woods. You should be able to find a used one for around $3000 or under. A new Woods 7500 unit sells for aroung $6000, and will dig a 7' 6" deep hole.
Hope this helps alittle.