navara gearboxes

Hi all, I need some help!, my 1989 navara (2.5 Diesel) has shat the gearbox again!, had to drive 100k's further in top gear with a trailer on!, i want to replace it altogether but i dont know alot about it. I am wondering if one of my spare steel case celica\supra boxes can be adapted to fit?, whats involved in swap?, or how do i fid a good replacement nissan box i can trust, I'm in Tasmania, Australia, any help or advice much appreciated, Thankyou...