I don't plan on changing the rubber mats on my '74 Spider, but will agree that a Spider with a carpet is much nicer. I've seen the IAP set in a 1972 Spider, and the fit was excellent. The only drawback that I see if you go with the carpet is if the car gets wet inside. With rubber mats it is really easy to get rid of the water, while not so with carpets. (Yes, I do drive my Spiders in the rain, and on occasion they have been parked with their tops down when it has started to rain...) If you are concerned about originality, the just know that the concours penalty for installing a carpet to replace the mats is 0.7 points, out of 100. It is your car, and if you prefer the carpet, by all means do what you want with it and enjoy your ownership.
Spider 74, 84 & 87
Milano 88 Verde
164 91S, 93L & 95Q
Member of the Alfa Romeo Owners Club USA