I recently did something simiar. I took an early engine and mated it to a late transmission. Here's what I learned that may be helpful to you.
First of all, I think you will find the 1980 car to have series glow plugs, while the '82 has pin type, parallel glow plugs, the glow plug relays are in different locations and totally incompatible. If you want to put the series glow plug engine in the parallel glow plug chassis, it will be a hassle to switch around the wiring and do a neat, clean and reliable job.
My transplant was a temporary one, my plan was to use the '80 engine temporarily while rebuilding the original engine to be replaced into the car later. So, my solution for the glow plug problem was sound, but most folks probably would not want to do something like this for a permanent installation. I put a momentary contact window switch in the panel at the top above the radio. I connected this switch to a Ford starter relay, because their cheap, rugged and easy to find anywhere. I then took a heavy cable from the relay to an 80 Amp glow plug fuse mounted behind the engine in the same location you would find on the '80 model. To start the car, I hold down the button with ignition on(I wired the relay primary side from a fuse in the box which is hot only with ignition on) for about 12 or 15 seconds in the summer and up to about 35 or 40 seconds in the winter.
Next issue. I would expect that there are different rear axle ratios in these cars. I would check this out. You can simply raise the rear of the car and count number of driveshaft revoutions for one rear wheel revolution. If they are different, you should consider changing the rear axle. You may have to change the driveshaft, crossmember and other components, but this should not be a big deal.
Changing the clutch linkage should not be a huge problem. The hydraulic nature should make this easier than changing a mechanical linkage. You may have to take the entire pedal assembly, I have no idea how much trouble this will be.
The next thing is mating the manual transmission engine to the automatic transmission. I ran into something similar mating an early engine to a late transmission that I believe directly applies here. There is an adapter plate bolted to the engine which then bolts to the appropriate transmission. If you remove the flywheel or flexplate, you can then remove the appropriate plate and transfer it to the other engine. I expect that both pilot bearings are the same, but it wouldn't hurt to check it closely.
I'm a manual transmission fanatic, so if it were me, I would go to whatever lengths to retain the 4 Speed, but that is of course a personal thing.
Also, these may have different A/C compressors. This should be no problem. Unbolt the compressor from the engine leaving it connected without breaking any AC lines, and wire it out of the way until you get the engine in place. If it is different, it will still bolt in place on the engine being transplanted.
These are easy cars to deal with, and everything is accessible, at least in the engine compartment.
If any other questions come up during the transplant, feel free to ask.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in