Well, two problems that I see. Your 240 is an '83, right? The '83 and '84 use the B23F engine, LH2.0 injection, and the 002 AMM (should be silver colored). From '85-'88, the 240 used the B230F with LH2.2 and the 007 AMM (black). '89 and later used LH2.4, with I believe the 016 AMM. Not compatible. If you decide to get an AMM from a junkyard, look at the VIN plate on dash for the 10th digit as 'D' or 'E' for '83 or '84. If you come across an '83 or '84 Turbo, it has K-jet and no AMM.
The other problem you might have is with the transmission. Up through 1985, the 240 had a mechanical cable driven speedometer, which connected to the trans. In '86, they went to an electronic speedo with the pickup on the rear end. If your donor car has a true 5 speed (no OD/5 button on top of the shift knob), then it's an M47, and it doesn't have the necessary cable connection to drive the mechanical speedo. The rear axle also won't have the necessary pickup for the electronic speedo, so you're SOL unless you can figure a way to hook a cable to the M47.
And before go out and buy an M46, you should also know that the driveshafts are not the same length between the M46 and M47, so you can't use the one from the donor car. 1986 was a crossover year with the older M46, but the newer electronic speedo. However, I don't know if an '86 M46 would have the cable speedo attachment or not.