Spider - 105 & 115 Series (1966-1994)
Alright, so international auto has the AR1237 shown here to fit on a 1978 Alfa.
Here is the link there:
Alfa Romeo and Fiat ANSA Performance Mufflers
And other sites like ebay and a few others have listed the AR1237 as made for a 86-93 Alfa Spider and the AR1007 as made for 78-85.