Congrats with your purchase of the Alfa Spider - sounds like a basicly good car that just needs some love and use.
There is not such a thing as matching numbers on Alfas of that era. If you write a mail to Marco Fazio you'll some facts. Here is my answer
Fra: ARCHIVIO STORICO ALFA ROMEO [mailto:ArchivioStorico@alfaromeo.com]
Sendt: 13. oktober 2006 07:59
Til: Erik Løye
Emne: JO: R: AR Spider 1750 Veloce
Dear Mister Løye,
with reference to your request we are informing you as follows.
According to our documentation files, the chassis number AR 1820173 originally corresponds to an Alfa Romeo 1750 Spider Veloce (105.57), manufactured on the 4th September 1970 and sold on the 28th September 1970 in Turin, Italy.
The body colour is ivory, with black skai interiors.
Automobilismo Storico Alfa Romeo
When I got it 10 years ago it was red (Farina Rosso), had got a rebuild engine of the right kind and lots of other stuff been repaired or renewed, but all geniue Alfa parts.
You can find lots of information on AR Spiders of the 105/115 series (like engine types for the various models) here www.http://veloce.se/