I dont think it matters much whether you start with a G or P or whatever SMEC harness.. to fit in an L, no matter what you use you will have some addditional length in places and certainly unused wires (which you could remove); unlesss you started with an 88+ TBI omni harness which would fit better but you still have to add the TII stuff (AIS & CAT) ; if you pulled the underdash harness also from an 88+ omni then you could plug & play that and eliminate the issues with the bulkhead connector
the hardest part will be adapting a pre-88 40 pin bulkhead connector to the SMEC harness.. and its not hard necessarily but you will need both vehicles wiring diagrams, then its just cut & splice, cut & splice ..
I am getting ready to do a similar swap but I think I will eliminate the bulkhead connector and direct wire all the under dash stuff right to the main engine harness.. which wont be *too* bad since I plan to remove all the wires that i dont need (including HVAC, wipers, AC, cruise, diagnostic plug, etc)