Done this, several times, Yes doable in a weekend if you are a decent to good wrench, dont have to change the cluster(no reason to), dont have to change diff. yes to rad, oil cooler, vacuum lines are so simple its not worth discussing. Ignition switch should be swapped so you can have the proper shutoff.

on a 380 SE, the driveshafts will be the same lenght. shifter is the same.

trust me on this one the 2.47 diff suits the 617, i averaged 28 MPG over 60K miles on 85 500 SEL with a 83 300 SD motor.

its worthwhile, Ive done it 3 times.

2 points to note, on my swap the tach did not work, I didnt swap clusters and wire it up,

second point, its bolt in except one of your engine shocks will be on the wrong side, and will not have a mount on the frame for it, I left mine off.

I didnt change front springs, and the wiring for a diesel is very straight forward.

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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