You wont need to swap X member just the motor mounts on the 89 from round to square and a few sensors on the motor but you must still use a <88 ECU and harness.I am in the process of doing the same thing. only 90 motor 86.5 chassis
a 7mgte is 7mgte regardless of year. only thing that needs to be switched are tps and the coolant temp sensor...i think it was that. one of the sensor on the thermostate housing. also motor mounts. the motor mounts swaps from one engine to the other perfectly...nothing else needed...no modding. in most cases, like mines, i just swapped out the whole thermostate housing with all the sensors, and my throttle body so i dont have to switch anything but motor mounts.