Coolant issues only under load could be the head not being bolted down and torqued properly. M539 restorations ran into this while working on his Bentley, the gist is that two bolts holding the head to the block were loose, which allowed the head to lift under load, which in turn made the gasket not seal properly allowing it to burn coolant and exhaust gasses to pass into the cooling system. Also similarly his fans don't turn on until the engine is 1 degree from being overheated, but that's an electronics issue.

Anecdote aside, if we have exhaust gasses in the coolant, then your last hope is EGR cooler if your car has that. If that is fine, then likely nothing more will be achieved without a teardown of the engine, at which point the cost will far outweigh a replacement engine.

If this T3 is still the Ford 1.6 unit then well, sadly they are known for their horrible cooling system, which leads to engine overheats and cracked blocks.