Actually if you concern is about the interlock system itself but not the buzzer warning systems, you could reach these three options:

1-Get the orange module used in Canada, which is a non interlocked control unit juts a sensors control box to still keep working the warning signals. Latelly they have floated around on $20-50 on ebay

2- jump out both yellow wires on reset button plug. No need to cut anything, just install a jumper wire between both yellow wires and even still plug the harness to the reset button.

3-you can also swap around both yellow wires on engine harness. This will send the straigh signal from ign switch to starter relay, instead reach the interlock box.

I have covered this extensivelly on FBBO and 3rd gen charger board, pics and diagrams included.

You could also get a 71 underdash harness for your 74 and match engine bay harness wires with it ( several wires were swaped around all allong the 71/74 production ). On 71 harness just will need to replace the terminals to the door jam switch and swap the wipers plug. Wires are the same between all 71/74, just 71s got diff prongs disposition ( 71 uses in fact 2 plugs for wiper switch instead one piece plug ). The advantage on use the 71 harness is it doesn't get any sensor or buzzer setup. No cut anything and no extra plugs hanging around or running to kick panel area for sensors, but just the optional light on warning buzzer. Seatbelt light works with courtesy ign light via time delay flasher/relay

then you can also unwrapp the underdash harness and remove all the wiring related to interlock system. Will need still to swap around the yellow wires on bulkhead/engine bay side... Being there done that., Once, when I still didn't have completed the interlock system to my 74 assembled in Vzla ( 74s in Vzla were wired just like 71s by the local assembly plant since interlock system wasn't a local requirement), I got a 74 harness what I modified to use on my car thinking on next upgrade. Once got it completed reisntalled everything back... yes I like all warning buzzers and interlock system working.

about the safety ground for starter relay. Even the interlock system gets that signal, it makes NOTHING to the ground safety system to the starter relay really. Interlock system just uses this signal to know when the car is geared or not to buzz/light on the seatbelt warning system, without interact with starter relay. Starter relay gets blocked just to the IGN yellow wire network.