I have never been able to successfully transplant a jackfruit after planting it in the ground. The taproot is very easily damaged and in every case, the tree has died. They also have a very low recovery rate from being root bound in pots as the tap root invariably wraps around the bottom of the pot. I always grow mine in deep forestry pots for that reason.
As far as grafting, I just use a cleft graft with the scion wrapped in parafilm. Assuming your scions have been properly prepped, you should get 90%+ success rate.