MIG welding isn't stronger or weaker than any other kind of welding. Although the engine mountings take a lot of force and vibration so it would need to be done well. You are probably looking at welding 5mm thick engine mountings to 3mm chassis at a right angle which will take a lot of oomph - Maybe 150 amps with a nicely prepared V at the end of the mountings.
I had a look through my photos for Landrover engine mounts and couldn't find any, but you might like the chassis swap we did one weekend.