1. how much the motor might cost?
2. how big of a pain is it to convert all the lines and tank to diesel?
3. how big of difference is the wiring harnesses?
4. is it worth doing?
I am "almost done" a similar swap myself... "Just" gotta hook up some wires to make the engine start. I had a JDM 80 with the 1HD-T and automatic transmission for three years and I decided it was time to get the steering wheel on the proper side and switch to a manual transmission while I'm at it. So I bought a FZJ80 in Portland and drove it home and began the swap...
To answer your questions:
1. Based on my research you can't get one for much less than $10k. Especially with shipping from Australia or Japan. Check
www.ebay.com.au and you'll see prices. Shipping in a crate will be at least $1000 with fees and taxes. ON THE OTHER HAND, I only paid $10,500 for my entire JDM truck here in Canada. And I had the pleasure of driving it for three years before I started the swap. I know you can't do this in the USA but.... What if you bought a JDM in Canada and cut it in half and brought each half down as a half-cut or something? Or just a U-Haul truck van full of parts? Having the donor truck in my back yard was EXTREMELY helpful. I can't imagine doing the swap without it... So many little parts that are diesel specific which you'd spend years searching for, or else have to jimmy rig.
2. Not much of a pain. The tanks are identical -- only the sending units are different. I left my gas tank in the truck and just drained it. A few drops of gasoline left over isn't going to kill the diesel engine. You should be able to get the diesel sending unit from Akella. The lines are different, and I did swap mine since I had them sitting here. The diesel has a larger diameter suction line (no fuel pump in sending unit) and a normal return line. Gas has a smaller diameter pressured line (fuel pump in sending unit), a return line, and a vent line that goes to the charcoal canister. The diesel tank does have the third fitting, but it just vents through a filter behind the tank and into the frame. I know other swaps that retained the gasser lines, but they had to add an in-line fuel pump to provide enough diesel to the engine at higher loads. I guess the smaller diameter pipe can't provide the flow without a little help, especially cold weather.
3. Wiring harnesses are completely different. Also I am doing LHD vs RHD. After pouring over EWDs, I think the easiest idea is.... Leave the harness connected to the engine, and leave the truck's harness hanging out of the firewall. Then splice as necessary. This is the stage I'm at. Obviously there are way fewer wires going to the diesel engine. I am just starting by making the starter spin, fuel cut solenoid open, then I'll move on to making the gauges work. You mentioned 1HD-FTE, but..... you really don't want to do this swap. The "E" means electronically controlled injection and I'm pretty sure this will be next to impossible. You can do a 1HD-FT (24 valve but no electronics). These engines seem to be extremely hard to find, so I just stuck with what was available locally... the 12 valve 1HD-T. I also have no idea how you'd make an electronically controlled transmission work...... But I wanted 5 speed so my only electronics is the switch for reverse lights! You could also find a donor truck with a mechanical automatic. (Mine had this but I sold it.)
4. Yes. If it will put a smile on your face every time you start the truck it is worth every penny! The diesel also has way more top end power and will burn about 60% as much fuel. I didn't realize just how gutless and thirsty the gasser was until I bought one and drove it for a couple months waiting for my 5 speed to arrive from Australia.
