You'll have some difficulties in doing this. The easiest thing in an engine swap is to make the cars "plug and play". You want all the sensors on the engine and connectors to match the wiring and computer that's part of the rest of the car. If you had a good runnign '85 engine I'd say do it. But one thing you'll run into is that the trans in the 79 has no hole for the crank speed sensor at the flywheel; and the flywheel has no holes for the sensor. So you'd either need to keep your old trans and swap your 90 flywheel to the new engine, or try something different.
Also, the B21F in the 79 will be a bit out of calibration for the 90 fuel injection setup- it's a smaller engine, so moves less air. While it would probably run, it may not ever run right.
Swapping ALL the 79 injection stuff into the 90 body is the other option but I know there are significant differences between the harnesses, not to mention the fuel pumps.
If they were mine, I'd advertise the 79 for parts, and if the 90 engine and trans seem to be dying I'd look for newer units. The junkyards have plenty in good running shape.
If the 79 is a 4spd + OD, it will fetch decent money in the classifieds. It's really only suitable for the pre 83 cars due to the mechnical speedo drive and the lack of the bellhousing hole prevents using it in 89-up cars without modification.
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE: 625K total