Welcome to the forum
As TT has said, you,ll need to fabricate engine mounts etc so adjusting the mounts for whatever gearbox is easy peasy, easier to do the mounts than having to refabricate the transmission tunnel once its all welded together ( tho this is still possible too )
Just be aware that the drivers footwell / pedal area will be smaller by about 25mm if you fabricate the chassis to accept the MT75 box.
cheers
andy