Steve you have a good old tractor that you have rebuilt the engine on.
Unless you need a cab i would have to keep the 706. You know what you have.
Tires are worth a lot. 40% rubber on the 770 means you may need new rubber soon.
18.4x34 rubber where i live are $800 plus labor and tube/each.
Personally i would love to have a nice 770 case with a cab. because I need a cab for health reasons.
The 706 should be 65-70 hp which i think is a little more than the 770 gas 55-65hp
Your 706 maybe worth $4000 but I think the 770 gas is similar in value.
You didnt say but is the 770 a powershift? Is the powershift OK?
Do you round bale hay or square bale hay. If you round bale you may need the extra HP of your 706.
Just some thoughts.