The dash harness may remain as long as it's the same sub-generation (pre 89 to pre89 OR 89+ to 89+). The tach has a couple different resistors depending on which ECU is used and for most it's easiest to replace the whole unit or the control board inside it.
As for modifying a GE harness to work with GTE electronics... FORGET IT... crazy idea and you'll spend years wondering why the car never started up again... it'd be easier to just build one from scratch.
The GTE's chassis harness has the connectors for the boost gauge sensor, IIRC and I'm not sure if the GE's body harness has the connector for the A/C push fan, but for the most part you're fine if you're going from manual to manual and same gen.
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