Hi there new here but I have a 2004 defender 90 td5 which has just had its engine swapped and everything running great but the coolant system is getting hot. Even though the gauge is sitting in the middle it’s overheating on the rad the water pump is working fine put a new thermostat on it bleed it up multiple times real slow with heater on and no air coming out of bleed screw. But go for a drive the coolant gets red hot and the top hose is empty and red hot. I’m struggling to think what it could be. Any advice much appreciated

Hello and welcome
best to ask over in the defender section of the forum
https://www.landyzone.co.uk/forum/defender-90-110-130.7/

Welcome.
Make sure you didn't install a thermostat for a V8 as they are rated higher at 88°C than a Td5 thermostat at 82°C.