It would be a good Idea.

They fail just about as often as the DS-II module. A spare Is always a wise decision.

I keep a spare DS-II module in my 1981 for example. But I travel a lot in the middle of nowhere too.

Just make sure you find a DS-I Green Grommet for a 1975. and Have It tested to make sure it's still good.

As for other differences, the major ones are the DS-I module and wiring, and the Instrument cluster wiring and printed circut. When you buy parts from now on, It might be easier to call it a 1975 for certain items, a 1977 for others. Not unlike the old Johnny Cash Song.