You can make your harness work with mods. Wiper motor wiring, brake light switch wiring etc. You have to shorten some wirea and make others longer, but it can be done.
A lot of Defender frames have all the mounts for either RHD or LHD, unless its a really early frame you shouldn't need to do any fab on the frame.
The upper dash pad is uni-sex, but the lower dash pad, the wiper motor cover, the insert and even the dash face is different. The gauges are the same, but not much else in the dash.
Most axles are uni-sex, you just use a LHD panhard rod and such and you'll need the RH swivel hosuing for the drag link pick up.
Don't forget a new heater. If you have all the parts it can be done in a week. Usually some small part will trip you up though and it takes longer.
I do not check PMs. Call or email if you need something.
ECR is not a parts source. We are not in the mail order business.