I did disco box too to plus the heater works pretty well. Mines to a 300tdi engine so maybe easier. Gatso did a custom lever housing so he could use the tdci tunnel and levers without cutting seatbox. Think he had a pneumatic hi low lever. I just cut seatbox and made a custom gear tunnel and plan on making a custom centre console/armresty/cubby boxy thing. and using D2 gaitors in it.
Wiring the lights and stuff in is pretty annoying and there's tons of wires in it you won't need from ecu and electronics. I got one with leccy window wiring in so be easier than if you're adding in. Gana use a central locking kit rarther than making the wiring in the tdci bulkhead work cos be easier. If you have a td5 now you might be able to use your system from that if lucky.
Make sure you get the heater air intake vent, you'll have to move your screen wash if it's in the same place as a 300tdi on td5 cos that vent.
If you go for puma bonnet too you need to offset your coolant resevoir again if it's in the same place as a 300tdi. Not had a td5 defender before just a disco
Make sure you have a tdi disco box you can't use a td5 disco box cos no speedo running gear I think.
I wish I'd made myself a new loom rarther than keeping the old ****ed about with one if yours has been messed with abit I recommend saving hassle and going new
You need to either swap the drum and handbreak or mod the disco one for def cable which you probably already know
Clocks I'm just using a peice of carbon fibre or ally with td5 clocks mounted on. Someone on here made a td5 set fit into the tdci surround though that looked awesome but like 150 quid for the tdci clocks to then chop up
Not sure if you needed new floor panels or not I did alot more than just the bulkhead so could have been from swapping a diff bit.
Don't let the heater control cables get rusty or bent it's really easy to do and they're 15 quid each and bastard to find
Don't forget to swap barrel or get new set for that and doors if want same