Yes, engine type AR00530 was fitted to the GT Junior. It was also fitted to the Spider Junior. The 8997 is the sequential serial number of the engine.
For 1966, the first year of production, Alfa assigned some 32,000 chassis numbers for the GT Jr, both LHD and RHD. Although it's doubtful that this many chassis were produced, it might be reasonable to assume that engine 8997 was produced in 1966.
Not everything Alfa is written in stone so the possibility exists that anything could have happened. Chassis were not assembled in numerical order nor were engines fitted in sequential order.
Personally, I think it quite unlikely that engine 8997 was factory fitted in a 1970s car but nothing is impossible. One must also keep in mind that starting in mid-1973, Alfa started producing and installing emission control engines in cars for export to countries with air pollution regulations. These engines were identified by the letter 'S' stamped in front of the engine's sequential number. This would seem rule out engine 8997 having been fitted to a mid-1973 on car destined for an emission control country.
1. The engine number AR00530*8997, as posted, is probably incomplete and therefore incorrect. The serial number after the asterisk is usually 5 digits, not 4. So, if it were an early number, it would be AR00530*08997 -- and if it is a later number, it would be AR00530*8997x (where "x" is a number between 0 and 9).
2. Fusi's production numbers shown in this thread indicate that engine number AR00530*08997 probably would date from about 1967 (not 1966) and AR00530*8997x would date from about 1971.