I am doing this to my T18 at the moment, you will need to find a custom flywheel, bell housing/gearbox adapter as they never came out manual, you will need to make custom mounts, hydraulic gearbox pedal box if you have a ke70, bigger diff, better suspension if you want it to handle half decent, bigger brakes, alot of work in it not just engine, box and loom.
You can get away with not modding the firewall but some people like to move it back because of the added weight, modify the transmission tunnel to suit gearbox if it is anything other than a W series gearbox and even then will still probably have to cut a new hole int the floor, forgot custom tailshaft.
Works out to be very expensive conversion but I think mine should work out to be cheaper than an SR20 conversion or similar but only because I can do everything myself (other than manufacture tyres to burn, haha.)
How much of the work can you do yourself? How much money do you have to spend? Do you want to do it 100% properly or do you just want to put one in because you saw another one with one in it.