I've done a similar project recently - converted a RHD HDJ81 to LHD. It was VERY involved and more difficult than i originally anticipated.
Converting the body only would be a lot easier - you wont have to worry about the frame/axle modifications. If you can reuse the dash wiring harness from the LHD truck, that will make it even easier.
The main difficult thing you'd have to do is convert the firewall and the cowl. There are dozens of little mounting brackets on the inside of the firewall that you would need to cut/weld from the donor truck to RHD body for all the things under the dash to bolt up to. As far as the holes for the AC lines/wiring/steering shaft, what i did was cut those sections out from the donor truck and weld them into the RHD firewall. Finally, you'd have to relocate the the cabin air intake in the cowl from one side to the other. And relocate windshield wipers. There will also be a million other small things that you'd need to do, but the above would be the most difficult part. Looking back at it now, if i was crazy enough to do it again, I'd pull the engine and replace the entire firewall with the LHD one. This would be easier to do than modifying the RHD firewall.