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The rear spoiler (along with the 3-piece front air dam) was a standard feature on the '78 Pace Cars only. It was not offered as an option for other '78s at that time.
The spoiler and air dam combo became a factory option in '79 (just as the thicker "pace car" style seats became a standard feature), but that didn't preclude owners of the '78s from getting them after the fact.
As far as your specific question, I've seen more '78 SA without the spoiler than with. But I don't recall anyone thinking that they looked "wrong".
Regarding non-special edition '78s with rear spoilers, to almost everyone else, they would look just like a '79 with that option. Considering that you've already got what looks like C4 seats, the addition of the spoiler shouldn't cause an avalanche of raised eyebrows.
My 78 was originally a silver anniversary car and came with the 3 piece front air dam and the rear spoiler. It is listed on the build sheet from GM as options on my car.