You're thinking along the right lines. The 90 gets the timing info from holes in a ring around the flexplate, or flywheel. The sensor is on a bracket poking through the top of the trans bell housing.
You need to put that bracket and sensor onto the 1987 block, because it won't have it, and install the 1990 flywheel, CLOCKED CORRECTLY TO TDC! The "start counting" pulse is a missing window in the ring, hence a longer pulse to the sensor, that starts the firing sequence.

Otherwise, exactly, you want to install all the '90's LH 2.4 sensors to the block and head of the 1987 engine.

Good luck with it!
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Rob Bareiss, New London CT ::: Roterande Fläkt Och Drivremmar!