I did one a few years ago, and right now Im doing it to a 500 SEC and a 380 SEC.

Its pretty plug and play, no cutting, no welding, depending on bodystyle your exhaust might fit as it.

My first one was a 83 300 SD into a 85 500 SEL. Ill briefly outline what I did, and im sure you can do it, as stated its much easier if you have both cars, and not just an engine laying around.

For starters, I swapped the engine and trans, I changed the output yolk on the SD trans to accept the flexdisk and driveshaft from the 500 SEL, I had to have the exhaust lengthened, I swapped the ignition switch from the diesel to the gaser, tranfering the gasser lock tumbler, I rigged up a pushbutton to activate the glow relay, (originally a switch, but I drained the battery so I opted for the pushbutton),
I left the cluster from the gasser, and lived without a glow light, wireing was pretty easy, oil pressure, water temp, switch to the gasser alternator, since its 85 amp.

I had no problem getting it registered, and i dont need a etest for a vehicle 20 years old.

I put 60K miles on it before selling it.

Only one engine shock will mount up unless you make a custom braket.

you will prefer the gearing in the gasser the 2.47 rear gears make for an awesome cruiser and are not that slow around town with the 5cyl.

Ill try to follow your progress to help out if i can.

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1984 Euro 300SDC, (4spd standard)
1986 Toyota Landcruiser Diesel HJ60 5spd X2

Gone but not forgotten (some sold, some stripped)
1983 300 SD, 1985 300 SD, 1983 240D, 1986 300 SDL, 1985 300 SDL, 1983 300 D, 1984 300 D, 1985 300SD, 1987 300 SDL, 1983 300 SD, 1985 300 TD Euro, 1983 380SEC, 1990 300 D, 1987 300D, 1982 300D, 1982 300D, 1994 E420, 1987 300 TD, 1987 300 D, 1984 300 D