Great little tractor and backhoe. I used several hoes that size on little tractors over a 20 year period. I did use them on the 3 pt successfully, but was always careful and had no problem. On the 3pt, I only used them for digging in the dirt; no roots, stumps, or stones.
However, you will get much better service from it if you use a subframe. I'd suspect that subframes for the JD750 are still in stock. They should be. It is a fine little tractor...wish I had one. And there were a ton of them made - sold both by JD and the similar models by Yanmar. So there should be subframes out there.
Be very careful when you hook it up to the 3pt to disable the tractor's draft control if it has it. It probably does; the similar size Yanmar does. And also be sure to hook up and adjust the positive stop on the mounting arms that is supposed to prevent the 3pt from rising up and smashing you between the backhoe seat and the back of the tractor. That particular defect in 3pt backhoes has killed a lot of good people with those 3pt backhoes before they made a design change mandatory. Be very careful with that. For years there was a memorial picture of a teen-ager killed that way at my JD dealer.
And in spite of knowing about it, it happened to me one day when I had the little 3pt hoe hooked up to the 3pt on our farm tractor. I happened to put down pressure on the backhoe bucket and that tripped the draft control. The 3pt arms sheared the stops on the backhoe mounting arms and the whole backhoe jerked up into the air so violently I couldn't figure out what was happening. Only had about one second to react, but luckily just barely bailed out the side in time and fell to the ground...... Shaking... The backhoe seat was crushed flat against the back of the tractor. Don't do that....
Of course if you use a subframe none of this will happen and the backhoe has several times more usable power.
rScotty