you can use the existing engine mounts ,exhaust etc, the problem may be the gearbox mounting side of things as you will have a mt75 set up! you will have to cut out the floor abit to fit the 4 speed ,but i think the best thing is to get hold of a 4 speed crossmember ,you will also have to have the propshaft alterd! on the gearbox end!
this has been done before so i know it will fit as ratwood on this forum has fitted a 3ltr v6 overdrive into his! ,it might be worth contacting him as he was talking about going diesel!
