Keep or sell the truck for parts, get yourself another Disco.
Sorry, there's really no better option unless you like to do engine swaps.

 

At this point, some would pour in a bottle of KSeal in the coolant as a last ditch effort and install a lower temp thermostat (maybe crack won't open or even open as much at lower temp). May work for some period of time. We also have a number of new members who have overheating shortly after buying one with such a treatment. But it might buy some time.

 

Moroso Ceramic Sealer
K&W Block Sealer

Both are made mainly of Liquid Glass (sodium silicate) which you can buy from a pharmacy if you really wanted. They are mostly discouraged on here, but its fairly common among the old flat head v8 and iron block crowd. Works just as well on aluminum. If the leak isn't too major it very well may seal it up, just follow the directions to a T. It may not last forever so I would have a plan B ready. It may buy you some time though.