3000gt automatic to manual conversion?

ive had so many problems all transmission related the mits rebuilds are worce then the originals. so i just want to convert it to a manual. how big of an issue is this and will i need to change anything with the ecu. they also make diffrent s for it. 4,5,and a 6 speed. i would like the 6 but that is and awd version, could i just put in the front wheel drive and keep the 6 speeds?